Daly-01.qxd
3/18/06
10:42 PM
Page 1
1 Air-conditioning fundamentals
The aim of this chapter is to:
●
●
●
●
Give an overview of the historical development of the heating and ventilation system and
introduction of the air-conditioning (A/C) system.
Provide the reader with a case study on the design and optimisation of an air-conditioning
(A/C) system.
Enable the reader to understand the fundamental principles and operation of the heating,
cooling, ventilation and air-conditioning system.
Introduce the possible replacement refrigerant/system to R134a.
1.1 History of automotive air-conditioning systems
The early history of transportation systems starts mainly with the horse drawn carriage.This was
eventually surpassed by the invention of the automobile. Early automobiles had cabin spaces
that were open to the outside environment. This means that the occupants had to adjust there
clothing to allow for different weather conditions. Closed cabin spaces were eventually introduced which required heating, cooling and ventilating to meet customer expectations. Early
heating systems included heating clay bricks and placing them inside the vehicle or using simple
fuel burners to add heat to the vehicle’s interior. Ventilation inside the vehicle was achieved
through opening or tilting windows or the windscreen; vents were added to doors and bulkhead
to improve air circulation and louvered panels were the equivalent to our modern air ducts. Air
flow was hard to control because it was dependent upon the vehicle speed and sometimes would
allow dirty, humid air which contained fumes from the engine compartment. Cooling could be as
simple as having a block of ice inside the vehicle and allowing it to melt! Eventually a number of
design issues were overcome, these included air vents at the base of the windscreen for natural
flow ventilation and electric motors to increase the flow at low speeds. Eventually heat exchangers were introduced which used either the heat from the exhaust system or water from the cooling system as a source, to heat the inside of the vehicle cabin. Early cabin cooling systems were
aftermarket sourced and worked on evaporative cooling. They consisted of a box or cylinder fitted to the window of the vehicle.The intake of the unit would allow air to enter from outside and
travel through a water soaked wire mesh grill and excelsior cone inside the unit.The water would
evaporate due to absorbing the heat in the air and travel through the outlet of the unit which
acted as a feed to the inside of the vehicle. The water was held in a reservoir inside the unit and
had to be topped up to keep the cone wet otherwise the unit would not operate. The air entering
the vehicle would be cool if the relative humidity of the air entering the unit was low. If the relative humidity of the air was high then the water could not evaporate.When the unit was working
effectively it would deliver cool saturated water vapour to the inside of the vehicle which raised
the humidity levels. These units were only really effective in countries with very low humidity.
Daly-01.qxd
3/18/06
2
10:42 PM
Page 2
Automotive Air-conditioning and Climate Control Systems
In 1939 Packard marketed the first mechanical automotive A/C system which worked on a
closed cycle. The system used a compressor, condenser, receiver drier and evaporator (fitted
inside the boot/trunk) to operate the system. The only system control was a blower switch.
Packard marketing campaign included:‘Forget the heat this summer in the only air-conditioned
car in the world.’ The major problem with the system was that the compressor operated continuously (had no clutch) and had to have the belt removed to disengage the system which was generally during the winter months. Over the period 1940–41 a number of manufacturers made vehicles
with A/C systems but these were in small volume and not designed for the masses. It wasn’t until
after World War II that Cadillac advertised a new feature for the A/C system that located the A/C
controls on the rear parcel shelf, which meant that the driver had to climb into the back seat to
switch the system off. This was still better than reaching under the bonnet/hood to remove the
drive belt. In 1954–55 Nash-Kelvinator introduced air-conditioning for the mass market. It was
an A/C unit that was compact and affordable with controls on the dash and an electric clutch.
The design and optimisation of an air-conditioning system
Case study – the air handling system
Experimental approach
In the past, the only way to evaluate a proposed air handling system design was to build a
prototype and test it in the laboratory.The air handling components were placed on a test stand,
conditioned air was supplied at the inlet and the airflow and temperature distribution at critical locations were measured. This approach takes a considerable amount of time and requires
the construction of expensive prototypes. In addition, it provides little or no understanding of
why a design performed the way it did. In particular, testing is unable to detect details of recirculating areas, turbulence, temperature stratification and constrictions that adversely impact
performance and pressure loss. In addition, the performance of the system usually needs to be
evaluated in many different configurations. For example, it sometimes is necessary to evaluate
the air handling system in different modes of operation – vent, floor, defrost and mixed – at
each of eight different temperature controls.
Modern methods of design
The design process of modern vehicle systems improved with the introduction of Computer
Aided Design (CAD), Computer Aided Engineering (CAE) and Computer Aided Manufacturing (CAM). CAD allows designs to be generated and visually appreciated on a computer.
Standard components can be shared among manufacturers and suppliers to ensure that components assemble correctly. Designs can be sent to clients for verification and feedback. Designs
can be modified and rechecked within short periods of time in a number of different formats,
e.g. an STL file (stereolithography). Complex parts and assemblies can often be manufactured
very quickly using rapid prototyping facilities (CAM). CAD also includes the facility to provide
virtual testing. This is generally provided using additional modules or add-ins converting CAD
to CAE.A number of secondary schools in the UK have the use of Solidworks as a CAD package for their technology departments which include add-in modules like Cosmos Works for
Finite Element Analysis and Computational Fluid Dynamics. Finite Element Analysis (FEA)
is basically mechanical stress analysis and Computational Fluid Dynamics (CFD) analyses the
flow of a fluid like air through or over complex geometry.These additional features are all computer based and use mathematical equations built into the software to predict variables like
the stress distribution of a component or assembly (FEA) or the flow of air through an air vent
(CFD). All these tests would have originally been carried out manually with continual adjustments being made to a model to optimise it.
Daly-01.qxd
3/18/06
10:42 PM
Page 3
Air-conditioning fundamentals
3
Figure 1.1 Computer generated model designed using CAD (without ducting and vents)
(© 2005 Visteon All Rights Reserved)
Figure 1.2 Air pressure loss predicted by CFD
(© 2005 Visteon All Rights Reserved)
The process
The A/C system begins life as an idea driven by consumer needs or sometimes government
legislation. This leads to a specification. The specification will include minimum performance
requirements, temperatures, control zones, flow rates etc. This will lead to a number of concept
designs. The designs will have a number of computer generated models which will be presented
as possible solutions to the original specification.These need to be tested for their performance.
Performance testing using CFD may include fluid velocity (air flow), pressure values and temperature distribution. Using CFD enables the analysis of fluid through very complex geometry and boundary conditions. The geometry typically includes ducts that expand and contract,
change from round to square cross-sections, go through complex curves throughout their length,
and have many branches and internal walls.
As part of the analysis, a designer may change the geometry of the system or the boundary
conditions such as the inlet velocity, flow rate etc. and view the effect on fluid flow patterns.
CFD is an efficient tool for generating parametric studies with the potential of significantly
reducing the amount of experimentation required to optimise the performance of a design.
A fan performance curve can be inputted into a model. Without this feature, the user has to
guess the flow within the enclosure, calculate the pressure using CFD and see if it matches the
Daly-01.qxd
3/18/06
4
10:42 PM
Page 4
Automotive Air-conditioning and Climate Control Systems
Figure 1.3 Streamlines showing flow field in an air handling system
(© 2005 Visteon All Rights Reserved)
Figure 1.4 Fan flow optimisation
(© 2005 Visteon All Rights Reserved)
fan’s characteristics. If the pressure doesn’t match, then another guess has to be made. Normally,
at least three iterations (test runs) are required to make a match.
The software enters the fan curve directly into the model. Each analysis run then interacts
with the fan curve to determine the precise operating conditions of the fan as part of the regular
Daly-01.qxd
3/18/06
10:42 PM
Page 5
Air-conditioning fundamentals
5
Figure 1.5 Improved fan design
(© 2005 Visteon All Rights Reserved)
Figure 1.6 Human modelling for temperature distribution
(© 2005 Visteon All Rights Reserved)
analysis. Using this technique, engineers can easily determine what type of fan is required to meet
air flow requirements within the vehicle, normally 158 cubic feet per minute (75 litres/second)
for heating and 300 cubic feet per minute (141.6 litres/second) for cooling.
As a typical example of improvements consider the typical design specification of the
HVAC system with respect to the temperature dial on the instrument panel. In other words,
moving the dial from position one to position two should have the same impact on temperature as moving from position two to position three. In the past, the linearity of the temperature
dial could not be estimated until full vehicle prototypes were constructed. At that point, changes
were costly and the testing data provided little or no input on what type of changes were required.
Daly-01.qxd
3/18/06
6
10:43 PM
Page 6
Automotive Air-conditioning and Climate Control Systems
Figure 1.7 Prototype HVAC unit for testing
(© 2005 Visteon All Rights Reserved)
Now, engineers can determine the linearity of a proposed design as soon as the solid model has
been created in a matter of days.They typically set up a series of analysis runs that evaluate eight
different temperature settings at each of the three HVAC system modes. In less than a week, they
can determine outlet air temperature at each setting.
Once all CFD modelling is complete the prototypes are made to ensure the physical models
operate as predicted by the computer models. The accuracy of simulated and actual system performance can vary up to 10–15%. Generally, lead times are reduced and designs can be evaluated
much quicker allowing more time to optimise their working performance.
1.2 Introduction to heating and ventilation
Heating and ventilation in automotive transport is not just a function of temperature control.
The safety of occupants to reduce driver fatigue, ensure good visibility and maintain comfort is
key to the successful design of such systems. A continual flow of air through the vehicles interior
reduces carbon dioxide levels, acts as a demister and prevents the build-up of odours. Carbon
dioxide in high concentration can cause a driver to be less responsive. There are recommended
ventilation rates which specify the number of times the internal cubic capacity (air space) of the
vehicle must be replaced per hour. Included in this calculation are the number of possible occupants and the internal volume of the vehicle. In some countries the performance of a heating and
ventilation system is governed by legislation. The heating and ventilation system combined with
an air-conditioning system provide a temperature range for occupants to select from.This can be
a real challenge due to some extreme weather conditions experienced across the globe. Often
auxiliary booster devices are required to provide additional ‘heating’ or ‘cooling’ of the interior.
The car heating system
Heat is a form of energy which means it cannot be destroyed. The principle of the heating and
ventilation system is to transfer enough heat from one point to another. The heater is a device
which heats the air entering or already inside the vehicle (recycled air). The heated air is then
directed to a combination of different places via a distribution of air ducts within the vehicle.
Daly-01.qxd
3/18/06
10:43 PM
Page 7
Air-conditioning fundamentals
7
Figure 1.8 Water cooling system
(with the agreement of Toyota (GB) PLC)
Figure 1.9 Heat exchanger
There are a number of different methods available to heat the air – exhaust heater, heat as a
by-product of combustion, electric heater etc. Generally motor vehicles use heat from combustion
which is transferred through water or air depending on whether the engine is water or air cooled.
If the vehicle is air cooled then a system of shrouds is used to direct the heat from the external
surface of the engine or exhaust or in some cases from the lubrication system towards the inside
of the vehicle.
Water-cooled engines
The engine has a water cooling system which is used to maintain engine temperature by transferring combustion heat (as a by-product of the combustion process) away from the combustion
chamber. The heated coolant is then carried from the combustion chamber through pipes to a
heat exchanger.
The heater exchanger (heater matrix)
The heat exchanger, often called the matrix, is situated inside the vehicle housed within the
heater assembly. It is designed to have a large surface area enabling air to pass over the surface
Daly-01.qxd
3/18/06
8
10:43 PM
Page 8
Automotive Air-conditioning and Climate Control Systems
of its fins. Fresh or recycled air (air from inside the vehicle) is directed under force over the surface of the heater core and then distributed via air and panel vents into the vehicle’s interior.
The heater core is made up of tubes and fins which are made from aluminium alloy and have
aluminium or plastic tanks attached to the core with inlet and outlet ports.
Heat control
Heat control is determined by the occupants of the vehicle.This is done by selecting the required
interior temperature which the occupants require via a control panel. The control panel will
either control components to allow more or less water to enter the heat exchanger or allow
more or less air to flow over its external surface.These two systems used to control the heater’s
thermal output are referred to as:
1. Water flow type.
2. Air mix type.
Water flow type
This system controls the amount of coolant flowing from the engine cooling system to the heat
exchanger using a control valve. The control valve (Figure 1.11) varies the flow of coolant
going inside the heat exchanger which in turn varies the temperature of the heater core.
Regulation in such a system can be difficult especially with the coolant flow and temperature
being dependent upon engine speed and load. The system does not respond immediately to
Heater core
Water valve
Figure 1.10 Heat controlled by a water valve
(with the agreement of Toyota (GB) PLC)
1. Control solenoid
2. Flow from engine
3. Flow back to engine
4. Flow from the heater core
5. Flow to the heater core
1
5
2
4
3
Figure 1.11 A solenoid operated water control valve
(reproduced with the kind permission of Ford Motor Company Limited)
Daly-01.qxd
3/18/06
10:43 PM
Page 9
Air-conditioning fundamentals
9
change, for example if a lower temperature is required to the interior then the control valve
will restrict the flow of coolant to the heater core. To achieve the reduced temperature the
heater core must lose the heat required to cool to the new selected temperature, thus giving off
heat. A benefit of regulating the heater core is that it allows the whole volume of air to flow
through the heater core itself, improving heating performance.
Air mix type
This system controls the volume of air allowed to flow over the surface of the heat exchanger
using an air mix/blend control door.The internal door directs air over or bypassing the heater core
depending on its position. The position is determined by the occupants who select a temperature
range from hot to cold. If a mid-range temperature is selected then the quantity of air will flow
over the heater core and a quantity will bypass the heater core.This air will then mix later in a mixing chamber to reach the final required temperature before leaving the heater assembly. The air
mix control doors are generally operated by Bowden cable, vacuum or electronic servo. The negative aspect of such a design is that the use of a mixing chamber means that the heater assembly
tends to be larger for the air mix type than the coolant controlled type. When not in use heat can
radiate from the heater core warming natural air flow which is transferred to the cabin space
although this can be overcome by using a shut-off solenoid to stop the flow of coolant when
maximum cooling effect is required. Positive aspects include quick response to changes in temperature and more accurate control of temperature variations.
Air distribution through the interior of the vehicle
A ventilator is a device used to direct air through the inside of a vehicle.There are generally two
types of ventilator used on a vehicle:
1. Natural flow ventilator.
2. Forced flow ventilator (blower).
Natural flow ventilator
This is created by air pressure outside of the vehicle caused by the forward motion of the vehicle.
As the vehicle moves in a forward direction positive and negative pressure is created on the vehicle’s surface due to its aerodynamic shape. Areas where positive pressure is created are ideal
places for air vents which allow air to enter the vehicle, travel through the interior and then exit
the vehicle via a vent often positioned in a negative pressure region.The air intake is positioned
Air mix control damper
Heater core
Figure 1.12 Heat controlled by an air mix control damper
(with the agreement of Toyota (GB) PLC)
Daly-01.qxd
3/18/06
10
10:43 PM
Page 10
Automotive Air-conditioning and Climate Control Systems
Figure 1.13 Positive and negative pressure across the surface of the vehicle
(with the agreement of Toyota (GB) PLC)
at the bottom of the windscreen where static pressure is high so air under pressure can flow into
the vehicle. There are a number of downsides to this position:
1. The engine compartment must be adequately sealed so no dissatisfying smells find there
way into the interior of the vehicle.
2. Air flow is proportionate to vehicle speed causing lack of flow at low speeds and possible
excessive noise and draughts at high speed.
This is generally reduced by only allowing a small pressure differential between the intake and
exhaust of air and adequate fan assistance. Air exhausts through outlets, which are mostly
located in a rear pillar hidden behind a trim panel or behind the rear bumper.
Air inlet and outlets
In Figure 1.14a to separate dirt particles and water from the air taken into the passenger compartment, the fresh air is fed through a cowl panel grille and pollen filter housing (2). This often
houses air filtration systems like pollen filters (1). The pollen filter is able to clean the fresh air
from smaller particles, such as dust and pollen which are able to get through the cowl panel grille.
The air outlets are arrangements of rubber flaps, mostly hidden behind the rear bumper or
behind a trim panel on the rear pillar. Because the pressure in the passenger compartment is
created by the blower motor and natural air flow, the air outlets open and air exchange can take
place. If there is no air flow through the vehicle interior, the flaps close to prevent exhaust fumes
from entering. If vehicle ventilation is unsatisfactory, check the flaps for freedom of movement.
In case of exhaust fumes reaching the luggage compartment, check the closing function of
the flaps and their tightness.
Forced air flow
In forced air ventilation systems an electric fan is fitted inside the vehicle. Fans are generally used
when vehicle speeds are low or comfort demands are high (demisting, heating and cooling).
Daly-01.qxd
3/18/06
10:43 PM
Page 11
Air-conditioning fundamentals
11
1
2
(a)
(b)
Figure 1.14 (a) Fresh air inlet; (b) Fresh air inlet Ford Fiesta
(reproduced with the kind permission of Ford Motor Company Limited)
The blower fan can force air over the heater core allowing the heat to be transferred to the air
which is distributed around the vehicle. Intake and outlet vents to the interior are generally
located in the same position as the natural flow ventilator. Figure 1.8 shows the position of the
fan (blower) within the system.
Fan design
All fans are driven by an electric motor. The motor can rotate at varying speeds depending on
the current supplied to it (Chapter 3). The fan allows the air flow to be adjusted according to
the requirements of the occupants. Fans are generally divided into axial and centrifugal flow
types. In axial type fans the air is drawn in and forced out parallel to the rotating axis. In the centrifugal type the air is blown in on the rotating axis and forced out perpendicular to the rotating
axis (the direction of centrifugal force). The shape of the vanes of the fans are often profiled to
maximise flow and volume and minimise size and noise.The blower is a dominant low-frequency
source of noise, while at higher frequencies, additional air flow noise sources exist. These
include high shear regions within the ducting, separation of flow due to flow obstructions, and the
exit flow from air vents. Flow optimisation can be achieved using CFD (Computational Fluid
Dynamics), which allows an engineer to analyse flow patterns and pressure regions within the
system and make adjustments on the size, shape and position of components in efforts to make
the system as aerodynamically efficient and quite as possible. Other efforts included noise
isolation through the use of padding or positioning sources outside of the interior.
Air filtration
Pollen filter
The filter is located in the heater assembly housing before the heat exchanger (Figure 1.15a
and b). Fibres in the filter prevent large particulates from entering the system and trap the
really small ones, the filter is electrostatically charged. Due to this electrostatic charge, the filter attracts particles like a magnet attracts iron filings. Besides removing visible particles, the
filter also removes pollen, spores and different types of dust etc. from the cabin air.
Carbon filter and germicidal lamp
An active carbon combination filter and/or a germicidal lamp are generally fitted as an option
in place of the pollen filter.The active carbon combination filter has the same advantages as the
Daly-01.qxd
3/18/06
12
(a)
10:43 PM
Page 12
Automotive Air-conditioning and Climate Control Systems
(b)
Figure 1.15 (a) Rear air flap. (b) Rear air flap Ford Fiesta (rubber flap removed)
(reproduced with the kind permission of Ford Motor Company Limited
Figure 1.16 Centrifugal type fan assembly
Figure 1.17 Axial type fan assembly
Daly-01.qxd
3/18/06
10:43 PM
Page 13
Air-conditioning fundamentals
13
Figure 1.18 Air filtration system with pollen, carbon filter and germicidal lamp
(with the agreement of Toyota (GB) PLC)
pollen filter plus an effective active carbon layer. The active carbon layer neutralises unpleasant odours and keeps the air free of ozone. It also reduces diesel exhaust fumes from entering the
interior of the vehicle. A germicidal lamp is used in air-conditioning systems to kill any bacteria
which enters or forms within the air filtration system.This also stops odours in the system through
the build-up of bacteria (see section 5.4).
Photo catalytic filter
Photo catalytic filters destroy pollutant gases and micro-organisms entering the vehicle. In less
than five minutes the entire cabin air can be purified.
Benefits:
1. Continuous protection against potentially harmful external/internal pollutants and from
discomforting odours.
2. Alleviation for allergy sufferers – micro-organisms causing allergies are destroyed.
3. Extended service life of filters – 2000 hours, equivalent to approximately five years’ average
vehicle use.
4. Complete destruction of pollutants compared to today’s carbon filters.
Daly-01.qxd
3/18/06
14
10:43 PM
Page 14
Automotive Air-conditioning and Climate Control Systems
(a)
(b)
Figure 1.19 (a) Air distribution showing panel, face and floor vents (b) Manual control panel
(reproduced with permission of Peugeot)
Photo catalytic oxidation converts toxic compounds like carbon monoxide and nitrous oxide into
benign constituents such as carbon dioxide and water without wearing out or losing its effectiveness. When light strikes titanium oxide, hydrogen peroxide (H2O2) and hydroxyl radicals
(OH) are formed. These two substances possess powerful oxidising properties and through
mutual interaction are able to decompose odorous substances into odourless carbon dioxide
and water. A powerful oxidative also removes bacteria and deactivates viruses.
Air quality sensing
An Air Quality Sensor (AQS) can be located in the main air inlet duct of the HVAC system.
When a threshold for carbon monoxide or nitrogen dioxide is reached, the AQS communicates
to the HVAC system to initiate the air recirculation mode. For more information see section 3.2.
Directing air flow and controlling temperature range can be manually selected by the occupants or electronically controlled via a control module. The heating system is designed to offer
a temperature range. Research into a comfort zone for passengers exists but is subjective due
to different nationalities that are acclimatised by the weather on their continents. The basic
heating and ventilation system control panel contains a temperature control knob and a number of air distribution options.
Air distribution unit
The air distribution unit is generally located under the instrument panel of the vehicle. Inside the
air distribution unit there is a system of ducts and mixing/directing doors. In addition the unit
houses the blower motor, the heater core and for vehicles with an air-conditioning system, the
evaporator.The filtered incoming air from the intake panel grille is induced by the blower motor
and is forced under centrifugal force to the air distribution unit.The air coming from the blower
is directed to the different air ducts through the moving doors in the air distribution unit. The
temperature is regulated by mixing warm and cold air. The air is then directed to different air
outlets/air nozzles and panel vents. There are basically two ways for the ventilation system to
take in air: fresh air from the outside and recirculated air from the interior. Therefore the air
distribution unit has two air inlets which are alternately closed by a door. Operating in recirculation mode allows to keep away unpleasant outside smells from the inside and it also improves
the cooling output of the air-conditioning system. When recirculation mode is switched on for a
longer period of time, the humidity level inside the vehicle will increase because of the moisture
content of the breath of the passengers. This can lead to fogging on the windows. Switching to
fresh air mode with air-conditioning system reduces the humidity of the inside of the vehicle.
Daly-01.qxd
3/18/06
10:43 PM
Page 15
Air-conditioning fundamentals
15
Face ducts
Air
intake
Recirculation door
(closed)
Air distribution door
(rotary door) and outlet
to panel
vents
Evaporator
Figure 1.20 Heater assembly
7
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
8
1
2
6
5
12
3
4
11
Air filtration
Air recirculation door
Blower motor and centrifugal fan
Heat exchanger
Temperature blend door
Air distribution door
Panel vent (not adjustable)
Face/head vent (adjustable)
Panel vent – rear passengers’ foot well
Ducting to passenger foot well
Flow of coolant
Control panel
10
9
Figure 1.21 Air recirculation
Simplified view of system components
Figure 1.21 shows the air intake door (recirculation door) is closed so no external air will enter
the vehicle except through a port which feeds the blower motor from the interior. The blower
is operating so air inside the vehicle will be recirculating around the vehicle’s interior.The heat
exchanger will still heat the air as required as long as there is a difference in temperature and
the occupants have selected so via the control panel, which will vary the position of the temperature blend door (2). While the air is recirculating there is a danger that water vapour will
condense on the inside the vehicle’s windscreen. This is affected by the following:
●
●
●
●
external air temperature;
interior temperature;
number of occupants;
relative humidity of the air inside the vehicle.
Daly-01.qxd
3/18/06
16
10:43 PM
Page 16
Automotive Air-conditioning and Climate Control Systems
Figure 1.22 Demisting position
Figure 1.23 Demisting and heating the occupants
If this occurs then air recirculation must stop and external air must enter the vehicle through
the air intake door. Recirculation of air is often selected when driving in polluted areas, e.g.
heavy traffic.
In the demisting position (Figure 1.22) the air from outside is moved under force from the
blower motor to the temperature blend door which is fully closed. This forces the total volume
of air to flow through the heater core where it will be heated and then directed by the top distribution door towards the windscreen and side windows. Note that no air is directed towards
the occupants. This allows the maximum volume of air to flow to the windscreen to aid the
demisting process. This is done through the evaporation of the condensed water droplets on
the screen (see section 1.3).
In Figure 1.23 the air intake door is fully open allowing external air to flow through to the
blower. The blower forces air towards the temperature blend door which is fully closed forcing all
the air to flow through the heater core.All the air flows through the heater core and is then directed
to the top distribution door where a portion is directed towards the windscreen and side windows
and the rest is directed to the foot vents which includes passengers in the rear of the vehicle.
Daly-01.qxd
3/18/06
10:43 PM
Page 17
Air-conditioning fundamentals
17
Mixing
chamber
Figure 1.24 Heat directed to occupants’ feet and face vents
Mixing
chamber
Figure 1.25 Air flow directed to face vents
The air intake door is fully open allowing air to flow through to the blower. The blower
forces air towards the temperature blend door. The blend door directs a volume of air towards
the heater core and the rest towards the distribution door allowing air to flow to the face vents.
The air going through the heater core is then directed towards the back of the blend door and
then the distribution door, where it is distributed by the feet vents (Figure 1.24).
There will be a temperature difference between the face vent and feet vent of approximately 7°C. This is due to humans feeling comfortable with their feet being warmer than their
head in cold conditions.
The air intake is fully open to allow air to flow through the blower. The blower forces air
towards the temperature blend door and depending on its position it will direct air straight to
the top distribution door and the face vents or it will direct a portion of the air towards the
heater core to raise the temperature of the interior and improve the comfort levels of the occupants (Figure 1.25). The interior temperature is generally controlled by the occupants via the
control panel. This selection offers the occupants fresh outside air straight to the head which is
beneficial in hot weather conditions removing heat from the occupants by convection. This
increases the occupants’ comfort, especially if perspiring, allowing the latent heat of evaporation to remove sweat producing rapid cooling, relative humidity permitting (see section 1.3).
Daly-01.qxd
3/18/06
18
10:43 PM
Page 18
Automotive Air-conditioning and Climate Control Systems
7
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
8
1
2
6
5
12
13
3
4
11
Air filtration
Air recirculation door
Blower motor and centrifugal fan
Heat exchanger
Temperature blend door
Air distribution door
Panel vent (not adjustable)
Face/Head vent (adjustable)
Panel vent – rear passengers’ foot well
Ducting to passenger foot well
Flow of coolant
Control panel
Evaporator (models with A/C)
10
9
Figure 1.26 System with evaporator fitted
System components with A/C (including evaporator)
Figure 1.26 illustrates the position of the evaporator in the heating and ventilation system. All
air passes through the evaporator irrespective of whether the system is operating. When the
A/C system is running the evaporator temperature is approximately 2–6°C (°F).This causes the
temperature of the air to reduce and moisture in the air to condense producing water droplets
on the evaporator’s surface.This reduces the moisture content (dehumidifying) of the air and also
helps to remove dirt particles (purifying) suspended in the air stream. The water covers the
surface of the evaporator trapping dirt particles and eventually dripping off the surface on to a
drain tray which directs the water to the outside of the vehicle.
Note – if the drain pipe becomes blocked then water will enter the inside of the vehicle.
Air vents
Air vents must be ergonomically designed to avoid draughts. Directional air vents generally have
three adjustments, up and down, left and right and open and closed. Circular vents are also used
which are free to rotate within a given circumference. The vents are generally used for face/head
heating. Panel air vents are fixed and cannot be adjusted.These are generally used for windscreen
and side screen demisting and floor heating.
Air diffuser system
The soft air diffusion system has been specifically designed to produce an evenly distributed
blanket of air that provides all vehicle occupants with the same high level of climatic comfort.
A range of diffuser systems are used within commercial heating and ventilation systems. They
are now being implemented within automotive climate controlled systems. There are a range
of diffuser types depending on the required air flow characteristics:
●
●
●
●
Linear diffusers provide continuous air flow across the length of an outlet. Air flow is quiet
and comfort increases while reducing drafts.
Mini-flow is used when quiet delivery and a low velocity of air is required.
Gentle-flow air jets with diameters ranging from 1/4 to 1/2 diffuse more quickly.
Super-flow air jets with diameters ranging from 1 to 6 can be used for a wide variety of
applications. The long throw of the air jets effectively propel air to greater distances.
Daly-01.qxd
3/18/06
10:43 PM
Page 19
Air-conditioning fundamentals
Air inlet control cable
19
Water valve control cable
Air flow control cable
Control panel
(w/lever)
Air mix control cable
Figure 1.27 Bowden cable used for air inlet, temperature blend and water flow control
(with the agreement of Toyota (GB) PLC)
Benefits of using diffusers
●
●
Alleviates discomfort – eliminates draughts from A/C system experienced by occupants of
front seats.
Improves climatic comfort for all occupants – air is distributed evenly throughout the cabin
and is particularly beneficial to back seat passengers.
Air door actuators
The doors are opened and closed by Bowden cables, pneumatic control motors or electrical
control motors. Manually regulated systems use Bowden cables in most cases. Automatic and
semi-automatic systems require control motors. These can be operated electrically or pneumatically. The pneumatic control motors are also actuated electrically by the solenoid valve in
the vacuum lines.
Bowden cable
Doors can be operated mechanically by Bowden cables. The rotation or sliding of a control
switch provides movement which is transmitted by cable to the doors.
Pneumatic control
Pneumatic control actuators consist of a diaphragm unit attached to an actuator rod. The
diaphragm has a spring acting on its surface holding it in position. To move the diaphragm the
spring pressure must be overcome. This is achieved by applying a vacuum via the vacuum connection (1).The vacuum is supplied by the engine inlet manifold often via the brake vacuum servo
connection (petrol engine) or brake vacuum pump (diesel engine).The vacuum creates a pressure
above the diaphragm which is lower than atmospheric pressure. The diaphragm housing has a
hole in its base allowing atmospheric pressure to act on the bottom surface of the diaphragm thus
the force of atmospheric pressure is used to overcome the spring tension.The rate of movement
is dependent on the spring tension and the difference in pressure between the upper (vacuum)
and lower (atmospheric) sections of the diaphragm. As the diaphragm moves the actuator rod
Daly-01.qxd
3/18/06
20
10:43 PM
Page 20
Automotive Air-conditioning and Climate Control Systems
1. Actuator rod
2. Diaphragm
3. Vacuum connection
3
2
1
ST/145/32
Figure 1.28 Recirculation air actuator vacuum operated
(reproduced with the kind permission of Ford Motor Company Limited)
moves as well. The actuator rod is attached to a door inside the HVAC unit. The door opens or
closes airways for recirculation (Figure 1.28). Pneumatic control actuators generally have two
positions – open and closed.They can be controlled by varying the vacuum applied using a variable orifice or by applying two connections of different diameter to apply different pressures. If
variable control is required it is easier to employ an electric motor.
A vacuum accumulator is generally employed to control the fluctuating pressure applied
to the diaphragm unit through the use of a non-return valve. A solenoid is also fitted to the
system to control fluctuating pressure when vacuum is applied. The valve can be operated
manually by means of switches or automatically by a control module.
This system has a number of drawbacks:
1. The vacuum is taken from the inlet manifold and uses critical energy otherwise used to aid
combustion.
2. If a leak is present in the system then combustion efficiency will be greatly reduced and
pollutants from exhaust emissions will increase.
3. The unit can only fully open a flap or fully close a flap if there is no position between these
points.
These units have generally been replaced with electric motors giving greater control.
Electronic control
Electrical control motors are used for fine adjustment of blend/distribution doors.These motors
are usually used for operating the temperature blend door and air distribution door, as this
door has to be moved proportionally. Systems with electronic temperature control often have
an integrated potentiometer in the control motor, which gives feedback to the control module
about the position of the door (for a full explanation see Chapter 3).
Classification of heating and ventilation systems by zone
A zone is an area of the internal space of the vehicle that can be cooled or heated to a specific
temperature. For example, a driver may feel hot due to their clothing and require cooling and
a passenger may feel cold and require heating. Both occupants have a set of controls to adjust
the temperature and ventilation rate of their personal space. Systems that have more than one
zone are generally electronically controlled. Heating and ventilation alone can be split for
Daly-01.qxd
3/18/06
10:43 PM
Page 21
Air-conditioning fundamentals
21
Figure 1.29 Electrical control motor and reduction gear
Figure 1.30 Dashboard installed HVAC system
(with the agreement of Toyota (GB) PLC)
Figure 1.31 Boot installed single zone HVAC system
(with the agreement of Toyota (GB) PLC)
zone control but generally if systems have this facility they include heating, ventilation and
air-conditioning (HVAC).
Dash HVAC
Installed under the dashboard with one single zone which is the interior space. The dashboard
type has the benefit of forcing cold air directly to the occupants enabling the cooling and heating effect to be felt to a much greater degree than the system’s capacity to cool or heat the
entire space. Example – the output at the air vent on an HVAC system might be 2°C which can
be blown directly on to the occupant’s head for immediate cooling. The interior space will only
cool to approximately 22°C.
Boot HVAC
Installed in the boot which has a large space available for the heating and cooling units. The
outlets are positioned at the back of the rear seat. Negative aspects of this design include loss
of boot space and cool air streams flowing from the rear of the vehicle.
Daly-01.qxd
3/18/06
22
10:43 PM
Page 22
Automotive Air-conditioning and Climate Control Systems
Figure 1.32 Dual zone dashboard installed HVAC system
(with the agreement of Toyota (GB) PLC)
Dual HVAC
Generally installed at the front of the vehicle under the dashboard and extended to the rear.
Dual systems can include up to three zones, driver, front passenger and the rear passengers.All
zones have a set of HVAC controls to select the desired level of comfort. This system is common on high specification vehicles and MPVs (Multi Passenger Vehicles) – vehicles with a high
number of passenger capacities.
Booster heater systems
Booster heating systems are generally used for the following reasons:
1.
2.
3.
4.
Large interior cabin space.
Efficient engine combustion with low heat output, additional heat input required.
Large interior space (MPV – Multiple Passenger Vehicle).
Vehicles operate in extreme weather conditions.
The benefits of such a system are as follows:
1. Improved visibility due to demist.
2. Shorter cold start period improving catalytic converter efficiency and less engine wear.
3. Improved passenger comfort.
PTC heaters
Booster heaters can be as simple as an additional water pump fitted to accurately control/boost
coolant through the heat exchanger fitted inside the vehicle to improve heating capacity, or a
separate unit which can provide additional heat input to the coolant or air distribution by
burning fuel (fuel heaters) or electricity (PTC – Positive Temperature Coefficient – heaters).
Booster heaters should not be confused with dual heating and air-conditioning systems. Dual
systems are extensions of the same system and provide heating and cooling control within designated zones inside a vehicle.
The PTC heater unit has an element mounted on a ceramic base and installed in the heater
casing. It directly heats up the air flow entering the passenger compartment.
The key characteristics of a PTC additional heater are:
●
●
●
●
rapid heating after starting the vehicle;
light and compact design;
the unit cannot overheat;
maintenance free.
Daly-01.qxd
3/18/06
10:43 PM
Page 23
Air-conditioning fundamentals
23
1
1. Heated core
2. Electrical connections
3. Connection cover
3
2
Figure 1.33 PTC heater unit
(reproduced with the kind permission of Ford Motor Company Limited)
1. Metallised ceramic plate
2. Aluminium elements
3. Frame
1
2
3
Figure 1.34 Heating element
(reproduced with the kind permission of Ford Motor Company Limited)
The PTC heater element consists of small metallised ceramic plates, which are layered alternately
along the unit core (1), see Figure 1.33 with aluminium radiator elements. These layers are held
together by spring elements in a frame. The aluminium elements provide the electrical contacts.
They also transfer heat to the passing heater air flow. In order to prevent electrical short circuits
due to metal foreign bodies, a heat-resistant plastic mesh with an aperture of 0.8 mm is located on
the heater element.The heater element is divided into separate heating circuits with a ratio of one
third to two thirds so that the heating power can be adapted to suit different requirements.
PTC heater elements act as a positive temperature coefficient resistor. This means that its
resistance value is relatively small at low temperatures and increases with higher temperatures. A high current flows initially when a voltage is applied to the cold PTC element, as a
result of which it heats up. As the temperature rises, so does the resistance. This results in a
reduced current draw. The time taken to stabilise the current is approximately 20 seconds. The
temperature of the additional heater depends on the rate of heat transfer to the surrounding
area. If the rate of heat transfer is good, for example, a cold low humidity condition will have
a greater temperature difference and allow for a greater rate of heat transfer, therefore the
resistance will remain low. Once the air has warmed and the heat transfer rate reduces the PTC
unit will start to increase in temperature due to the inability to give up heat. This will cause the
Daly-01.qxd
3/18/06
24
10:43 PM
Page 24
Automotive Air-conditioning and Climate Control Systems
1. Heater unit
2. Fuel metering unit
3. Coolant hoses to rear heat exchanger
2
3
1
Figure 1.35 Diesel heating booster system location
(reproduced with the kind permission of Ford Motor Company Limited)
2
3
4
1
1.
2.
3.
4.
5.
6.
Coolant inlet from the engine cooling system
Coolant outlet to rear heat exchanger
Fuel feed from metering unit
Electrical connection
Air intake hose
Exhaust with built-in damper
5
6
Figure 1.36 Booster unit
(reproduced with the kind permission of Ford Motor Company Limited)
resistance on the unit to increase and thus reduce the current flow. The reduced current flow
maintains or reduces the temperature. If the opposite occurs and the heater manages to give
up heat to the surrounding air then the unit will cool and the resistance will reduce. This will
increase current flow through the unit and increase the unit’s temperature. As a result of these
specific resistance characteristics, it is not possible for the PTC element to overheat. The maximum surface temperature is around 165°C.
The unit is only operated at low ambient temperatures (15°C, supplied by air temperature
sensor) and when insufficient heat can be supplied via the coolant-based heating system (73°C,
supplied by coolant temperature sensor) and low alternator (generator) loadings.The PTC heater
consumes a great deal of current so it can only be operated when the engine is running and its
operation is phased. Phased operation means that the unit is treated like a three bar electric
heater. One bar at a time is switched on so the load on the alternator (generator) is progressive
and not sudden. The load will also affect the idle speed and emissions.
The heating power is around 1 kW when fully operating.This places additional loading on the
vehicle electrical system. Vehicles with PTC additional heaters are fitted with a more powerful
generator.
Daly-01.qxd
3/18/06
10:43 PM
Page 25
Air-conditioning fundamentals
1
2
3
A
4
5
1.
2.
3.
4.
5.
6.
7.
A.
B.
C.
D.
E.
B
6
25
Combustion blower
Flame sensor
Glow plug
Temperature sensor
Overheating sensor
Control module
Metering pump
Coolant outlet
Coolant inlet
Fresh air supply
Fuel return line
Exhaust
D
C
E
7
Figure 1.37 Booster heater operation
(reproduced with the kind permission of Ford Motor Company Limited)
Diesel fuel booster system
If a vehicle cannot provide adequate heated coolant to a heat exchanger then it may use a fuel
booster to provide the additional heat input. There are a number of manufacturers using such
a system due to the ultra efficient diesel combustion system and the large interior space that
requires heating. This is often between 1 and 3 kW. At maximum output the fuel penalty is
0.38 litres/hour and the operating temperature is between 40 and 80°C. Once above 80°C the
unit shuts down. Such units can provide a variable output often dependent upon the measured
temperature of the coolant flowing to the heat exchanger and the exterior air temperature.
Such a unit may use air or coolant to transfer heat from the unit to the interior of the vehicle.
All the starting, regulating and run-on functions are controlled fully automatically.The main
unit is divided into four main sections – heat exchanger, combustion chamber, fan assembly
and control module. Figure 1.37 shows the unit with the coolant inlet (B) and outlet (A) on top
of the unit. This feeds the heat exchanger section which allows coolant to flow around the outside of the core. The inside of the core is the combustion chamber which contains a glow plug
(3), flame sensor (2) and fuel and air inlet. The combustion is initiated by a glow plug fed with
fuel which is quantified by the metering unit (7). The glow plug is switched off after a certain
time when the flame has stabilised. The fan assembly/combustion blower (1) provides a fresh
supply of air to the combustion chamber. After combustion the heat flows from the inside of
the heat exchanger which is then conducted through the exchanger’s walls by the coolant.
Because the heating power output depends on coolant temperature, sensors are fitted to the
heat exchanger housing monitoring coolant temperature to and from the unit. This information is used to govern the unit’s heat output to the vehicle’s heat exchanger. The output temperature sensor and combustion sensor are used for failsafe purposes in case the unit fails, e.g.
overheats. All information is monitored by the ECU (6) which has the ability to communicate
Daly-01.qxd
3/18/06
26
10:43 PM
Page 26
Automotive Air-conditioning and Climate Control Systems
with diagnostic equipment. Because the system is electronically controlled, integration into an
existing HVAC (Heating, Ventilation and Air-Conditioning) system can be easily achieved.
The booster heater is switched off due to a malfunction if:
1.
2.
3.
4.
5.
no ignition takes place after a second attempt at starting (after 90 seconds);
the flame is extinguished during operation and a fresh attempt at starting fails;
the overheating sensor responds in the event of overheating (125°C);
the voltage is over- or undershot;
the glow plug, metering pump or temperature sensors or fresh air blower are faulty.
The booster heater can be tested as soon as the ignition is switched on. A fault code can be
read out during the diagnostic check.
Use of an A/C system as a heat pump
With the use of highly efficient diesel engines, hybrid vehicles, and electric fuel cell technology,
water-based cooling systems often do not offer the heating capacity for passenger compartments. The vapour compression cycle A/C system can be used as a heat pump. This process is
explained in section 1.5.
1.3 The basic theory of cooling
An air-conditioner is a generic term for a unit which maintains air within a given space at a
comfortable temperature and humidity. To achieve this, an air-conditioning unit must have a
heater, cooler, moisture controller and a ventilator.
The principle of an HVAC system:
●
●
●
●
●
heater – adding heat by transferring it;
cooler – removing heat by transferring it;
humidity – removing or adding moisture;
purification – by filtration;
ventilation – air movement through the vehicle.
The HVAC system creates a comfort zone for the occupants which can be adjusted within a
range. Not all humans desire exactly the same environment, variations occur due to different
continents, countries, cultures, gender, age or simply due to the type/amount of clothing being
worn when inside the vehicle. The system must provide a way of controlling the climate inside
the vehicle which is generally referred to as ‘climate control’.
Heat
Heat is a basic form of kinetic energy which cannot be destroyed; only it can be converted to
or from other forms of energy. In accordance with scientific laws all heat will travel from a hot
to a cold surface until the temperatures have equalised. The rate at which heat will travel is
dependent upon the difference is temperature between a hot (more energetic molecular movement) and cold (less energetic molecular movement) area. The SI unit for heat energy is the
joule. Other measurements include calories and BTU (British Thermal Units).The effects of heat
energy are measured using temperature.
Daly-01.qxd
3/18/06
10:43 PM
Page 27
Air-conditioning fundamentals
Boiling point
of water
Freezing point
of water
Absolute zero
150
423
100
373
212
50
323
122
0
273
32
150
123
238
200
73
328
273.15
0
459.67
Celsius scale
[°C]
Kelvin scale
[K]
Fahrenheit scale
[°F]
27
Temperature scales
Figure 1.38 Temperature comparison
(with the agreement of Toyota (GB) PLC)
Heat intensity
The SI unit for heat intensity is the kelvin, the derived unit is Celsius or Fahrenheit. The kelvin
scale is a theoretical scale based on the laws of thermodynamics using absolute zero as the
start of its scale instead of zero as used with °C. Scientists state that a temperature called
‘absolute zero’ is the point at which all heat is removed from an object or substance (a complete absence of molecular movement). The intensity of heat can be measured using a thermometer. This only gives the heat intensity of a substance and not the heat quantity.
The scales on the chart in Figure 1.38 show the boiling point and freezing point of water. It
must be noted that this is only true when it occurs at sea level – 101.3 kPa (1.013 bar). Heat
intensity within a vehicle for comfort should be between 21 and 27°C (65 and 80°F).
Sensible heat
Heat which causes a change in temperature is called sensible heat. As previously stated it can
be ‘sensed’ using a thermometer or pyrometer. Theory tells us that by adding heat to a liquid
such as water their will be a proportional increase in its temperature which can be measured
on a thermometer scale, e.g.:
The amount of heat required to raise the temperature of 1 kg of water by 1°C is 4.2 kJ.
For example:
420 kJ of heat must be applied to 1 kg of water at 0°C to bring it up to the boiling point of
100°C.
Conversely, the same amount of heat must be removed from the boiling water to cool it down
to freezing point.
Daly-01.qxd
3/18/06
28
10:43 PM
Page 28
Automotive Air-conditioning and Climate Control Systems
Alternatively:
1 BTU heat quantity changes the temperature of 1 lb water by 1°F.
Specific heat capacity
Different substances absorb different amounts of heat to cause the same increase in temperature. Specific heat capacity is used to measure the amount of heat required to cause a change
in the temperature. The basic unit for specific heat capacity is the joule per kilogram kelvin
(J/kg K). Different materials have different specific heat capacity values.
Latent heat and a change of state
Latent heat (hidden heat) is the heat energy required to change the state of a substance without changing its temperature. Heat can have a direct effect on substances when they change
state. Evaporation is the term used when enough heat is absorbed by a substance to cause it to
change into a vapour. Condensation is when enough heat is removed from a vapour causing it
to change to a liquid. When a change of state occurs a great deal of energy is either absorbed
or released. This is called the latent heat of vaporisation and the latent heat of condensation.
Example:
2260 kg of latent heat is absorbed when 2 kg of water at 100°C changes state from a liquid to
a vapour.
970 BTU of latent heat is absorbed when 1 Ib of water at 212°F changes state from a liquid to
a vapour.
This will occur without any change in the thermometer reading.
If ice is heated it will reach 0°C (32°F) then start to melt.This means you will have liquid and
a solid existing together. The more heat you add the more the ice will melt with no increase in
temperature until no ice exists, then the water will start to increase in temperature (sensible
heat). When water reaches 100°C (212°F), any more heat energy added will result in some or
all the liquid changing into a vapour. The amount of vapour produced depends on the heat
energy available and the pressure above it.
When a substance changes state it can absorb hundreds of times more energy than when it
is just increasing in heat intensity. This latent heat is the common process used to transfer heat
from the interior of the vehicle to the exterior. The key to successful cooling is to get the liquid
or vapour to the point where it wants to condense or evaporate at the right place. To do this we
manipulate the system pressure.
The function of the heating system is to increase or reduce heat input to the inside of the
vehicle. A typical automotive combustion engine is only about 30% efficient, which means
only 30% of the fuel delivered to the combustion engine is converted into usable energy. Some
of this energy is transferred to the cooling system which is where the heating system will obtain
its heat source for the inside of the vehicle.The rest is lost through exhaust gases and radiation.
Heat can be transferred using one or a combination of three processes – conduction,
convection or radiation.
Conduction, convection and radiation
Direct heat movement from one molecule to the next through a substance here the direct heating
of a bar one end will travel through the molecular structure to the other end. Some materials
3/18/06
10:43 PM
Page 29
Liquid to gas
Latent
heat
Sensible
heat
Latent
heat
29
Gas
Liquid
Sensible
heat
Solid to liquid
Solid
Air-conditioning fundamentals
Temperature
Daly-01.qxd
Figure 1.39 Sensible heat and latent heat
(with the agreement of Toyota (GB) PLC)
Figure 1.40 Conduction
(with the agreement of Toyota (GB) PLC)
are excellent conductors of heat – aluminium, copper – and other materials act more like
insulators – polymers (plastics).
Convection is heat movement through a medium like a liquid in a saucepan. Convection is
a continuous movement of medium and heat. The medium, liquid or gas, moves and releases
heat to the surrounding areas. When heating occurs in a liquid or gas expansion occurs and
parts of the substance become lighter than other parts which contain less heat. Natural
convection currents occur in any substance which is not heated evenly.
Heat can travel through heat rays and pass from one location to another without warming the
air they travel through. An example is ultraviolet radiation which travels from the sun. Radiant
heat can travel from a warmer object like the sun to a cooler object like the earth’s surface. The
surface colour and texture affect the heat emitted and absorbed. Colour is not as important as
texture; dark rough surfaces make better heat collectors than light smooth surfaces.
Daly-01.qxd
3/18/06
30
10:43 PM
Page 30
Automotive Air-conditioning and Climate Control Systems
Figure 1.41 Convection
(with the agreement of Toyota (GB) PLC)
Figure 1.42 Radiation
(with the agreement of Toyota (GB) PLC)
From the engine to the interior
Convection
Convection occurs when material, such as an engine, passes heat to the cooling system of the
vehicle. As the potential energy of the fuel is converted to mechanical and heat energy by the
engine combustion process, the heat of the engine must be removed. The liquid in the cooling
system is pumped through the engine, and the convection process transfers engine heat to the
liquid. The cooling system liquid then takes this heated coolant to the radiator. The metal heat
exchanger uses the conduction process to remove the heat from the liquid coolant and to the
exchanger fins.The radiator fins then pass the heat of the radiator to the passing air flow through
the heat exchanger.
Enthalpy
Enthalpy is the measure of the usable energy content of a substance. When a liquid increases
in temperature it also increases in enthalpy. When the liquid changes into a vapour through
Daly-01.qxd
3/18/06
10:43 PM
Page 31
Air-conditioning fundamentals
31
Table 1.1 Relationship between vacuum and temperature at which water boils
Boiling point
of water °C
48.9
43.3
37.8
32.2
26.7
21.1
15.6
10
4.4
1.1
6.7
12.2
15
Boiling point
of water °F
kPa
bar
120.02
109.94
100.04
89.96
80.06
69.98
60.08
50
39.92
30.02
19.94
10.04
5
89.60
92.50
94.80
96.50
97.80
98.80
99.60
100.40
100.60
100.80
101.00
101.10
101.20
0.896
0.925
0.948
0.965
0.978
0.988
0.996
1.004
1.006
1.008
1.01
1.011
1.012
latent heat of evaporation it does not increase in temperature but does increase in enthalpy
because the energy within the substance increases.
Pressure
Pressure is defined as the force exerted on a unit area by a solid, liquid or gas. The SI unit used
to indicate pressure is the pascal (Pa). Other units of pressure are pounds per square inch (psi),
kilograms of force per centimetre squared (kgf/cm2), atmospheres (atm) and millimetres of
mercury (mmHg). Atmospheric pressure at sea level is 101.325 kPa (14.6 psi). This is generally
shown on a gauge as zero (gauge pressure) unless it is a gauge which measures atmospheric
pressure. This means that absolute pressure is atmospheric pressure plus gauge pressure.
Previously it was discussed that water changes state at 100°C. This is if the liquid is at sea
level under atmospheric pressure. If you reduce the pressure on the liquid by moving it above
sea level or apply a vacuum to it then the boiling point is lowered. If you increase the pressure
applied to the liquid by moving it below sea level or specifically applying a pressure on it, then
the boiling point is raised.
Table 1.1 shows that if a deep vacuum (1 bar vacuum) is created in a closed system like an
A/C system water will boil at 10°C (50°F). This enables technicians to remove moisture from
the A/C system using a vacuum pump (refer to Chapter 4). The chart also presents the importance of creating a deep enough vacuum under cold ambient conditions.
If we place a liquid that will readily evaporate at atmospheric pressure inside the box but
close the tap, the pressure will build up and increase the boiling point of the liquid (e.g. at a
pressure of 5 bar water only boils at 152°C). When the tap is open the pressure will suddenly
decrease and the liquid will readily evaporate lowering the temperature by absorbing the heat
inside the box.
Critical temperature and pressure
There is a critical temperature which is the maximum point at which a gas can be condensed
and a liquid can be vaporised by raising the pressure. Refrigerants R134a and R12 have critical
Daly-01.qxd
3/18/06
32
10:43 PM
Page 32
Automotive Air-conditioning and Climate Control Systems
Figure 1.43 Liquid removing heat by changing state
(with the agreement of Toyota (GB) PLC)
pressures and temperatures that dictate the maximum pressure/temperature they can be subjected to. If an air-conditioning system operates below the critical points of its refrigerant then
they are termed subcritical systems. If they exceed there critical pressure/temperatures then
they are termed transcritical systems.
Refrigerants
Refrigerants are the working fluid of the A/C system. An ideal refrigerant would have the
following properties:
1.
2.
3.
4.
5.
6.
7.
Zero ozone depleting potential and zero global warming potential.
Low boiling point.
High critical pressure and temperature point.
Miscible with oil and remain chemically stable.
Non-toxic, non-flammable.
Non-corrosive to metal, rubber, plastics.
Cheap to produce, use and dispose.
Refrigerant CFC12 – dichlorodifluoromethane
R12 is a CFC (Chloro Fluoro Carbon).The refrigerant consists of chlorine, fluorine and carbon,
and has the chemical symbol CCL2F2. It was used for many years from the early development of
A/C systems up to the mid-1990s when it was progressively phased out leading to a total ban on
1 January 2001 due to its properties which deplete the ozone and contribute to global warming
(see Chapter 6, section 6.1). A benefit of R12, when it was originally designed, was its ability to
withstand high pressures and temperatures (critical temperature and pressure point) without
deteriorating compared to other refrigerants that were around at that time. R12 mixes well with
mineral oil which circulates around an A/C system. It is non-toxic in small quantities although
does displace oxygen and is odourless in concentrations of less than 20%. R12 can also be
clean/recycled.You must not burn/heat R12 to a high temperature (300°C) with a naked flame
because a chemical reaction takes place and phosgene gas is produced. A lethal concentration
of phosgene is 0.004% per volume.
Daly-01.qxd
3/18/06
10:43 PM
Page 33
Air-conditioning fundamentals
33
R12 properties:
1.
2.
3.
4.
5.
6.
7.
It is miscible with mineral oils.
It does not attack metals or rubber.
It is not explosive.
It is odourless (in concentrations of less than 20%).
It is not toxic (except in contact with naked flames or hot surfaces).
It readily absorbs moisture.
It is an environmentally harmful CFC gas (containing chlorine which destroys the
atmospheric ozone layer).
8. It is heavier than air when gaseous, hence the danger of suffocation.
Refrigerant HFC134a – tetrafluoroethane
R134a is a known substitute for R12. R134a is an HFC (Hydro Fluoro Carbon). The refrigerant consists of hydrogen, fluorine and carbon, and its chemical symbol is CH2FCF3. Because
the refrigerant has no chlorine it does not deplete the ozone. R134a is non-toxic, non-corrosive
and does contribute to global warming; it is not miscible with mineral oil so synthetic oil, called
PAG (Poly Alkaline Glycol), was developed. PAG oil is hygroscopic and absorbs moisture rapidly
which means when in use you must ensure that the container is resealed as quickly as possible.
R134a cannot be mix with R12 and is not quite as efficient at high pressures and temperatures.
R134a can be cleaned and recycled.
R12 and R134a have different size molecules, the R12 molecule is larger. This means the
quantity of refrigerant required for an R134a system is higher than an R12. This also requires
the flexible hoses and seals including oil to be replaced with compatible R134a components if
a conversion from R12 to R134a is required (see Chapter 5). R134a contributes to global warming and will eventually be replaced, certainly within Europe, with another cooling medium
which is reported to be less harmful to the environment. R134a is not a drop-in replacement for
R12 A/C systems. A number of changes are required to the system components to allow an R12
system to use R134a as a cooling medium (see Chapter 5).
Properties of R134a:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
It is only miscible with synthetic polyalkylglycol (PAG) lubricants, not with
mineral oils.
It does not attack metals.
It attacks certain plastics, so only use special seals suitable for R134a.
It is explosive.
It is odourless.
It is not toxic in low concentrations.
It readily absorbs moisture.
It is inflammable.
It is heavier than air when gaseous, hence the danger of suffocation near the ground.
Refrigerant blends
The use of alternative refrigerants to R134a and R12 are not accepted by OEM standards.
Manufacturers only use the approved R134a refrigerant in vehicle A/C systems. The US has a
range of Snap Approved Refrigerants which can be use as ‘alternatives’ to R12. If blends are
used then separate approved service units and acessories must be used to avoid contamination.
Strict procedures must be adhered to with regard to record keeping, barrier hose fitment, high
pressure release device replacement, service connectors fitment, strict labelling requirements,
Daly-01.qxd
3/18/06
34
10:43 PM
Page 34
Automotive Air-conditioning and Climate Control Systems
Table 1.2 Substitutes acceptable subject to use conditions for CFC-12 in MVACs
Substitute
HCFC22
HFC134a
R406A
GHG-X4, R414A (HCFC Blend Xi)
Hot Shot, R414B (HCFC
Blend Omicron)
FRIGC FR-12, (HCFC
Blend Beta), R416A
Free Zone (HCFC Blend Delta)
Freeze 12
GHG-X5
GHG-HP (HCFC Blend Lambda)
Ikon 12, Ikon A (Blend Zeta)
SP34E
Stirling Cycle
CO2
RS-24
Evaporative Cooling
Trade name
GHG, GHG-X3, GHG-12, McCool,
Autofrost X3
GHG-X4, Autofrost, Chill-it,
Autofrost X4
Hot Shot, Kar Kool
Retrofit/new
R, N (buses only)
R, N
R, N
R, N
R, N
FRIGC FR-12
R, N
Free Zone/RB-276
Freeze 12
GHG-X5
GHG-HP
Ikon 12
SP34E
R,
R,
R,
R,
R,
R,
N
N
R,
N
RS-24
N
N
N
N
N
N
N
Key: R Retrofit uses, N New uses
oil replacement and possible seal replacement. Blends currently cannot be recycled using an
approved service machine. This means refrigerant has to be sent back to the supplier to be
recycled. Blends are compounds made of other refrigerants, R22, R134a, etc. They are either
azeotrope or zeotrope. Azeotrope has a single boiling point while zeotrope are a boiling point
range. Zeotrope blends’ boiling point range starts when the lighter elements start to boil and
ends when the heavier elements boil off. If a leak occurs the blends’ lighter elements will vaporise and escape leaving the heavier elements. This is called fractionising, which changes the
characteristics of the blend.
Note – R134a is the only refrigerant that should be used as a replacement for R12.
Retrofitting must be carried out in accordance with SAE requirements (see legislation).
Substitutes
Substitutes are reviewed on the basis of ozone depletion potential, global warming potential,
toxicity, flammability, and exposure potential. Lists of acceptable and unacceptable substitutes
are updated by the EPA (Environmental Protection Agency) several times each year.
Refrigerant service connectors
To prevent accidental mixing of the refrigerants the SAE (Society of Automotive Engineers)
developed guidelines for different service valve connectors for R12 and R134a. If refrigerants
are mixed severe damage will occur to the system. R12 uses threaded connections and R134a
quick release couplings.
Daly-01.qxd
3/18/06
10:43 PM
Page 35
Air-conditioning fundamentals
1
35
1. R12 high side connector (3/160 flare 3/8-24 threads)
2. R12 low side connector (1/40 flare 7/16-20 threads)
2
Figure 1.44 R12 threaded service connectors
(reproduced with the kind permission of Ford Motor Company Limited)
1
2
1. R134a high side connector (16 mm quick coupler)
2. R134a low side connector (13 mm quick coupler)
Figure 1.45 R134a quick release type service connectors
(reproduced with the kind permission of Ford Motor Company Limited)
Figure 1.46 R134a blue low pressure connector quick release coupling
(courtesy of Autoclimate)
The service connector forms a valve with the valve core screwed into it.This valve allows the
connection of a pressure gauge, A/C machine or control switches/sensors to be removed without draining the system. These are called Schrader type spindle valves. They are similar in
design to tyre valves. The needle is held in the closed position by spring force.
Before the coupling is fitted the shut-off valve (blue) must be closed ensuring the valve is
not open. Once connected the valve is opened to obtain a reading on the service unit. Blue is
allocated to the low pressure side or suction side and red is allocated to the high pressure side
or discharge side.
Figure 1.47 shows a threading low pressure R12 connector with ball valve preventing refrigerant loss.
Daly-01.qxd
3/18/06
36
10:43 PM
Page 36
Automotive Air-conditioning and Climate Control Systems
Figure 1.47 R12 low pressure connector 1/4” SAE Schrader valve 45°
end fitting with Goodyear GY5 barrier hose
(courtesy of Autoclimate)
Figure 1.48 Schrader valve
(courtesy of Autoclimate)
Figure 1.49 Valve core remover and installer with shut-off valves
(courtesy of Autoclimate)
The protective cap prevents the valve getting dirty and also provides an additional seal when
the system is working. The protective caps must be screwed on again after the system is filled.
The service connector valves must seal completely. To check, apply a few drops of compressor
oil to the needle. If bubbles are formed, the valve is leaking and the valve core must be renewed.
Tools are available to remove Schrader valves without draining the system, providing there
is sufficient space.
Hose material
A/C systems are designed to use as little flexible hoses as possible due to leakage. Aluminium
extruded tubing is generally used with the exception of the compressor which uses flexible
hoses because it is connected to the engine. Modern hoses for R134a use an inner lining of
nylon due to the size of the R134a molecule and to reduce moisture ingression. Covering the
nylon is an external tubing of Neoprene. Polyester braid is used as reinforcement with a final
Daly-01.qxd
3/18/06
10:43 PM
Page 37
Air-conditioning fundamentals
37
Table 1.3 Comparative data
Unit
Chemical name
Colour of storage bottle
Thread on container
Connector size/type
Lubricant
Desiccant
Boiling point at 1 bar
Critical temperature
Critical pressure
Molar mass
Liquid density 20°C
Ozone reduction factor
Hot house factor
Coldness gain
Cooling efficiency
°C
°C
bar
g/mol
kg/l
kj/m3
R12
R134a
Dichlorodifluoromethane
White
7/16
1/4 flare 7/16-20 threads
3/16 flare 3/8-24 threads
Mineral oil
XH-5
29.8
112
41.48
120.92
1.33
0.96
3.1
1495
4.3
Tetrafluoroethane
Light blue
8/16
Low quick couple 13 mm
High quick couple 16 mm
PAG
XH-7 or XH-9
26.5
101.15
40.68
102.03
1.23
0.0
0.3
1455
4.2
covering of PVC (Polyvinyl Chloride). R12 hose is constructed in the same way but without
the nylon lining due to the larger size molecule. Electric compressors may remove the need for
any flexible hoses. The government and environmental groups are placing increased pressure
on zero leak rates.
OEM (Original Equipment Manufacturers) will always advise you to use the correct refrigerant for the correct system. Deviation from the original set-up should only occur if you are
converting an R12 system to an R134a.
The pressure/temperature relationship of R134a
The graph in Figure 1.50 shows the pressure/temperature curve for refrigerant R134a.
The graph shows the refrigerant to be in a gaseous/vapour state above the curve and a liquid
below the curve.
1. The refrigerant is in a gaseous/vapour state and if the temperature is kept constant and the
pressure is increased then the refrigerant will condense into a liquid.
2. If the pressure is kept constant and the temperature is reduced then the refrigerant can be
condensed into a liquid.
3. If the temperature is kept constant and the pressure is reduced then the refrigerant will
evaporate into liquid/vapour.
4. If the pressure is kept constant and the temperature is increased then the refrigerant will
evaporate into liquid/vapour.
Comfort – humidity
Humidity is the term used to describe the wetness or dryness of air. The air around us contains
a percentage of water vapour. Humans perspire (sweat) which evaporates and cools the surface
of the skin by convection. If humans are in hot humid conditions then this makes us feel sweaty,
Daly-01.qxd
3/18/06
38
10:43 PM
Page 38
Automotive Air-conditioning and Climate Control Systems
Figure 1.50 Pressure/temperature curve R134a
(with the agreement of Toyota (GB) PLC)
uncomfortable, anxious and can induce stress. A fan to force air over the occupant of a vehicle
can improve the evaporation rate and improve comfort.
There are generally two ways to measure humidity, relative humidity and absolute humidity. Relative humidity is the most common measurement and tells us how much water vapour
by weight the air actually contains compared to how much it could contain at that given temperature. As an example, if the relative humidity is 50%, the air could hold twice as much
vapour as it does at that given temperature. The amount of vapour that the air can hold
changes with its temperature. If the air warms up then it can hold more vapour which would
reduce the relative humidity because it could hold more vapour than what it was actually carrying. If the air cools then its relative humidity reduces because it can now hold less.
Absolute humidity is the amount (by weight) of vapour that the air contains compared with
the amount of dry air. When air becomes saturated with water and then cools the relative
humidity will eventually (depending on the rate of cooling) become 100%. The temperature is
called the ‘dew point’ of air for the absolute humidity. If the air cools any further then the
vapour it contains will condense.
The significance of this information is the control of the humidity within the A/C system.
Humidity is controlled by the surface area of the evaporator and the volume and flow of air travelling through it. The cold surface of the evaporator causes the moisture in the air to condense
and cover the surface in water droplets. This reduces the moisture content thus drying the air
and improving comfort. Relative humidity for comfort levels is generally about 60%.
Dry bulb temperature
This is the temperature indicated by an ordinary thermometer used to measure air temperature.
Wet bulb temperature
In a wet bulb thermometer the heat sensitive bulb of a glass tube thermometer is wrapped in
a gauze, one end is suspended in a water container to allow the water to be drawn upwards by
capillary action and moisten the bulb. The moisture robs a percentage of the heat surrounding
the bulb which is dependent upon how easily the water can evaporate. The temperature that is
registered is referred to as the ‘wet bulb temperature’. Some equipment suppliers sell as an
accessory a probe with a wet sock which attaches to a multi-meter allowing the electronic measurement of temperature. The sock can be removed to obtain the dry bulb temperature. As
Daly-01.qxd
3/18/06
10:43 PM
Page 39
Air-conditioning fundamentals
39
Figure 1.51 Humidity graph
(with the agreement of Toyota (GB) PLC)
discussed, relative humidity is measured by comparing the wet bulb temperature against the dry
bulb temperature. Instruments that contain both measuring devices are called psychrometers.
As an example – in Figure 1.52 the graph shows the wet bulb temperature 19.5°C (follow diagonal line), dry bulb temperature 25°C (horizontal line) and relative humidity 60% (point
where both intersect).
After measuring the dry and wet bulb temperatures of the air entering the evaporator and
the air exiting the centre vent inside the vehicle the graph can be used to calculate the relative
humidity and compare the two results ensuring good evaporator performance. Example, relative humidity of the ambient air entering the HVAC unit – 70%, relative humidity of the air exiting the centre vents inside the vehicle – 50%.
Humidity sensor
See Chapter 3, section 3.2.
1.4 Vapour compression refrigeration
Currently the most common cycle used in automotive applications is the vapour compression
cycle, e.g. R12 and R134a closed systems. To understand the vapour compression cycle and
Daly-01.qxd
3/18/06
40
10:43 PM
Page 40
Automotive Air-conditioning and Climate Control Systems
60
(67)
19.5
25
(77)
Figure 1.52 Psychometric chart
(with the agreement of Toyota (GB) PLC)
other cycles it is important to understand the types of changes that a refrigerant goes through
when used in an A/C system.This is explained using a pressure/enthalpy diagram (Figure 1.53).
A refrigerant in the ‘subcooled’ liquid region point a is at a temperature which is below its
boiling point. If heat is continually added while maintaining constant pressure, the refrigerant’s temperature and enthalpy will increase. Its state will eventually approach ‘saturated
liquid’ point b.This is where the liquid will start to vaporise.As the heat is continually added the
liquid vaporises and continues to increase in enthalpy but not increase in temperature. The ‘saturated liquid’ vaporises until it becomes a ‘saturated vapour’ point b–c. The ‘saturated vapour’
at point c has no liquid because is has completely vaporised. The heat that is absorbed through
the transition from ‘saturated liquid’ to ‘saturated vapour’ is called the latent heat of evaporation (heat is absorbed without any increase in temperature). With additional heat still available
to the ‘saturated vapour’ the temperature increases causing the refrigerant to become ‘superheated vapour’ point d.
At this point the heat addition causes an increase in temperature. The saturated liquid and
saturated vapour curves meet at a point called the ‘critical point’.This point has a corresponding
critical pressure and critical temperature.Above the critical pressure the refrigerant is in a state
called the ‘supercritical region’. The supercritical region is where heat addition or removal does
not cause a distinctive liquid vapour phase transition.
The ideal vapour compression cycle
The following applies the vapour compression cycle as an ideal cycle within an automotive
A/C system.The Figure 1.54 pressure/enthalpy diagram shows the beginning of the cycle as the
refrigerant enters the compressor as a ‘saturated vapour’ at point 1. The refrigerant is compressed adiabatically (the compressor is 100% efficient and no heat is removed by the process)
and becomes a ‘superheated vapour’ due to the increase in pressure, temperature and enthalpy
10:43 PM
Page 41
Air-conditioning fundamentals
Constant temperature lines
(Temperature increases to the right)
Supercritical region
Critical point
b
a
Pressure
3/18/06
d
c
Subcooled
liquid region
Superheated
vapour region
Mixed liquidvapour region
Saturated
liquid line
Saturated
vapour line
Enthalpy
Figure 1.53 Pressure/enthalpy diagram
(Still Awaiting Permission)
Constant temperature lines
Supercritical region
Critical point
Pressure
Daly-01.qxd
2a
3
2
Subcooled
liquid region
Superheated
vapour region
4
Saturated
liquid line
1
Saturated
vapour line
Enthalpy
Figure 1.54 Ideal vapour compression cycle
(Still Awaiting Permission)
41
Daly-01.qxd
3/18/06
42
10:43 PM
Page 42
Automotive Air-conditioning and Climate Control Systems
as shown by point 2. The refrigerant by this point is above the temperature of the outside air.
The refrigerant leaves the compressor and enters the condenser (heat exchanger). The condenser allows the heat to transfer to the outside air effectively removing enough heat to
change into a saturated vapour from a superheated vapour point 2a. This has lowered the temperature of the refrigerant. Now the refrigerant is a ‘saturated vapour’ the pressure and temperature are kept constant but heat is still being removed and only the enthalpy continues to
decrease.The vapour begins to condense to liquid (latent heat of condensation). Condensation
continues until all the vapour is a ‘saturated liquid’ point 3. The refrigerant leaves the condenser as a saturated liquid and travels to an expansion valve or fixed orifice tube.The refrigerant
now undergoes an ‘isenthalpic’ expansion process (constant enthalpy). The process significantly reduces the temperature and pressure of the refrigerant while the enthalpy remains the
same.A small amount of liquid refrigerant (flash gas) vaporises during expansion but most of the
refrigerant is liquid at a temperature lower than that of the outside air (air inside the vehicle
or entering the vehicle) point 4. The refrigerant flows through the evaporator which acts as a
heat exchanger that transfers heat from the air flowing through its fins to the refrigerant flowing through its coils.The refrigerant absorbs the heat increasing in enthalpy while the temperature
and pressure remain the same. The liquid refrigerant vaporises until it becomes a ‘saturated
vapour’. The saturated vapour then travels to the compressor to start the cycle again.
The real world operation
The real world operation of the A/C system deviates from the ideal cycle. It is difficult for
condensation and evaporation to end exactly on the liquid/vapour saturation lines. This is particularly difficult due to the fact that the system has to operate under so many varying conditions. To ensure an acceptable performance under all loads the condensers are designed to
‘subcool’ the refrigerant to a certain amount to ensure that only liquid refrigerant flows to the
expansion device for optimum performance (also one of the jobs of the receiver drier). If
vapour flows to the expansion device it reduces the flow of refrigerant significantly.
Evaporators are generally designed to slightly ‘superheat’ (TXV systems) the refrigerant to
ensure that only vapour flows to the compressor and no liquid (except for oil circulation 3%).
There are also pressure drops across components like the condenser and evaporator which
cause deviation from the ideal constant pressure process. Fixed Orifice Valve (FOV) systems
use a fixed orifice diameter which is designed for optimum flow at high compressor and
vehicle speeds. Poor performance at idle conditions can occur where there is a possibility of
the evaporator being excessively flooded. Some flooding is advantageous with the FOV system. This is to increase cooling capacity and reduce ‘hot spots’, which are areas of reduced heat
transfer caused by poor refrigerant distribution. Too much flooding reduces compressor
performance.
1.5 Alternatives cycles
Pressure from the European Union for more environmentally friendly A/C systems has forced
manufacturers to look for alternative refrigerants or technologies for HVAC units. A great
deal of controversy exists on which technology the EU, US and the automotive industry wants
to supersede R134a or in fact just improve the current R134a into a leak-free system.The EU will
phase out R134a from 2011 with a complete ban on its use by 2014 to 2017 (dates to be finalised).
Possible alternatives are within this section although it seems inevitable that CO2-based A/C
systems will replace the R134a system which is currently used.
3/18/06
10:43 PM
Page 43
Air-conditioning fundamentals
43
A list of possible alternative are:
●
●
●
●
●
●
CO2-based system (R744);
absorption refrigeration;
secondary loop system (HFC152);
gas refrigeration (R729);
evaporative cooling;
thermo electric cooling (Peltier effect).
The production of R744 (CO2)-based HVAC systems will be included on vehicles being mass
produced by 2008. The following information has been provided to aid the reader in predicting what refrigerant and accompanying technology will be implemented to supersede R134a.
The CO2 (R744)-based refrigeration cycle (transcritical system)
Refrigeration and air-conditioning systems where the cycle incurs temperatures and pressures
both above and below the refrigerant’s critical point are often called transcritical systems.
Transcritical systems are somewhat similar to the subcritical systems described above although
they do have some different components.
Figure 1.55 illustrates the transcritical vapour compression process. It begins when the superheated refrigerant enters the compressor at point 1. Its pressure, temperature and enthalpy are
increased until it leaves the compressor at point 2 located in the supercritical region. Next the
refrigerant enters the gas cooler whose function is to transfer heat from the fluid to the environment. Unlike the condensing process in the subcritical system, the refrigerant has not undergone a distinct phase change when it leaves the gas cooler at point 3. Note that this gas cooling
Constant temperature Lines
4
3
2
Critical point
Pressure
Daly-01.qxd
Subcooled
liquid region
6
5
1
Superheated
vapor region
Saturated
liquid line
Saturated
vapor line
Enthalpy
Figure 1.55 Pressure/enthalpy graph transcritical system using R744 (CO2)
(Still Awaiting Permission)
Daly-01.qxd
3/18/06
44
10:43 PM
Page 44
Automotive Air-conditioning and Climate Control Systems
process does not occur at constant temperature. The cooled gas then enters an internal heat
exchanger (sometimes called a ‘suction line heat exchanger’), which transfers heat to that portion
of the refrigerant that is just about to enter the compressor.
This results in additional cooling of the refrigerant to point 4 on the figure, improving performance at high ambient temperatures. From there, the flow undergoes a constant-enthalpy
expansion process that decreases its temperature and pressure until it exits at point 5 in the
mixed liquid/vapour region, at temperature and pressure well below the critical values. Next, the
refrigerant enters an evaporator where it absorbs heat from the cooled space and its enthalpy
and vapour fraction gradually increase until it exits at point 6. Finally, the flow enters the internal
heat exchanger where it absorbs more heat, until it is ready to enter the compressor again at
point 1 to repeat the cycle.
Note – the R744 cycle will also work in the subcritical region (i.e. some condensation in
the gas cooler will take place) in case the ambient temperature is considerably lower than
the critical temperature of R744, which is about 31°C.
System operation
Figure 1.56 shows a R744 closed loop A/C system capable of acting as a heat pump.The increased
use of highly efficient diesel engines, particularly direct injection models as presently occurring
Figure 1.56 System layout for A/C and heat pump operation
(© 2005 Visteon All Rights Reserved).
Daly-01.qxd
3/18/06
10:43 PM
Page 45
Air-conditioning fundamentals
45
in Europe, and the anticipated increase in the use of hybrid vehicles, means that engine coolant
will no longer have the customary temperatures and capacities for acceptable passenger compartment heating and window defrosting/demisting operations. However, a heat pump can be
used to heat to the passenger compartment boosted to temperature levels to which vehicle occupants are accustomed.
The diagram (also reproduced in colour in the plate section) shows arrows in blue which
represent refrigerant flow when in A/C mode and red arrows when in heat pump mode.
A/C operation (blue arrows)
1. Compressor
Superheated refrigerant enters the compressor (temperature 30°C, pressure 35 bar).
The refrigerant is compressed increasing its pressure, temperature and enthalpy (130 bar,
160°C). The given values represent a high load point.
2. The gas cooler (replaces the condenser)
The refrigerant enters the gas cooler (via the active switching valve), upon entering the
gas cooler the superheated gas allows heat to be transferred to the walls of the gas cooler
and air travelling through it. The refrigerant does not go through a distinct phase change
although its temperature (at the gas cooler outlet a few kelvin above inlet
air temperature, for example 40°C for a 35°C ambient temperature) and enthalpy reduce.
The refrigerant is still operating above its critical point.
3. The accumulator/internal heat exchanger
The refrigerant flows to the high pressure side of the internal heat exchanger which
removes heat by transferring it to the refrigerant that is about to enter the compressor.
This again reduces the refrigerant’s temperature (30°C).
4. Electronic expansion valve
The refrigerant flows to the electronically controlled expansion device which creates a
large pressure drop promoting the constant-enthalpy expansion process. The reduced
pressure and temperature of the refrigerant now allows the device to operate below its
critical point. The refrigerant is now a mixture of liquid and a flash vapour having the
ability to change state with additional heat input.
5. Evaporator
The refrigerant flows into the evaporator absorbing heat through evaporation until it
exits the evaporator as a saturated vapour.
6/7. Accumulator/Internal heat exchanger
The refrigerant flows (through the passive switching valve) to the accumulator/internal
heat exchanger. This component combines the accumulator and internal heat exchanger
functionalities into one part. The internal heat exchanger section enables the refrigerant
to become slightly superheated. The accumulator portion separates the liquid and
gaseous phase, stores the unused liquid refrigerant and allows the compressor oil together
with the gaseous refrigerant to return to the compressor for lubrication.
8. Slightly superheated refrigerant flows back to the suction side of the compressor and the
process repeats.
Note – temperatures and pressures are approximate and are dependent on system load.
Heating operation (red arrows)
Heating is achieved by directing the flow of heated refrigerant to a secondary gas cooler
positioned inside the vehicle which heats the incoming/recycled air and is distributed through
Daly-01.qxd
3/18/06
46
10:43 PM
Page 46
Automotive Air-conditioning and Climate Control Systems
conventional ducting. The refrigerant then flows to the accumulator/heat exchanger and external gas cooler which is positioned at the front of the vehicle.
1. Compressor
For an assumed ambient temperature of 18°C, the refrigerant enters the compressor at
a temperature of about 20°C and a pressure of 18 bar. The refrigerant will be
compressed to about 90 bar at 90°C.
2. The secondary gas cooler
The active valve will divert the flow that – in an A/C cycle – would usually go to the gas
cooler, to the secondary gas cooler. Upon entering the gas cooler the superheated/high
temperature gas allows heat to be transferred to the walls of the gas cooler and the air
travelling through it. The refrigerant is still operating above its critical point. Both
temperature and enthalpy reduce.
3/4. The electronic expansion device
The refrigerant flows to the electronically controlled expansion device which creates a
large pressure drop promoting the constant-enthalpy expansion process. The reduced
pressure and temperature of the refrigerant allows it to operate below its critical point.
The refrigerant is now a liquid and flash vapour having the ability to change state with
additional heat input. Note that for a heat pump cycle the expansion device needs
to be a bi-directional type as flow enters from both sides.
5. The accumulator/internal heat exchanger
The internal heat exchanger (IHX) section has no duties in the heat pump cycle
(both the former high and low pressure sides of the IHX are now located on the low
pressure side of the cycle) and is only a passage for the refrigerant.
6. The gas cooler
The refrigerant flows from the accumulator/internal heat exchanger to the gas cooler,
which in fact now acts as the evaporator for the heat pump cycle, where it will absorb
more heat to change from a saturated vapour to become slightly superheated.
7. The accumulator/heat exchanger
The passive valve collects the flow of the heat pump branch of the circuit and directs the
flow to the accumulator/internal heat exchanger. Again, the IHX is pretty much without
function, but the accumulator section acts exactly as in A/C operation.
8. The compressor
The refrigerant flows from the accumulator/heat exchanger back to the compressor
and the process repeats itself.
Note – pressure and temperature figures depend on system load.
Heat pump operation will only be used at very cold ambient temperatures allowing for faster
defrost and/interior warm-up. It does not replace the normal heater core and is just a supplement. Thus the mixed A/C/heater mode is still available, but there will be no combined A/C/heat
pump operation (as both functions rely on the same refrigerant circuit). A/C will continue to
be used for dehumidification in warmer ambient temperatures or during comfort drive, assisted
by the normal heater core for reheating. Removing particles is the duty of the air filter that is
located upstream of the evaporator.
R744 properties
R744 has a corrosive effect on polymers so metal pipes are used. R744 is non-flammable and
relatively cheap compared to R134a. CO2 is also easier to recycle than R134a.
Daly-01.qxd
3/18/06
10:43 PM
Page 47
Air-conditioning fundamentals
47
Table 1.4 Properties of R744
R744
Operational mode
High pressure
Low pressure
Max gas temperature
A/C line diameters
A/C line fittings type
Flexible hose material
Compressor displacement (cm3)
Compressor housing diameter (mm)
Expansion device
Transcritical
60–140
35–50
Up to 180°C
10–12
Axial metal
Steel
20–33
100–120
Electronic valve or mechanical orifice
Front end heat exchanger
Gas cooler
(Specifications and descriptions contained in this book were in effect at the time of publication. Visteon reserves the right to discontinue any equipment or change specifications without
notice and without incurring obligation (07/05).)
Component information
Sanden and LuK have been assisting with research and development of compressor designs
for R744 systems.
●
Research in 2000 was based on compressor LVT (30–36 cm3), the parts are interchangeable
with compressor VDA (160 cm3) – R134a.
Maximum pressures and temperatures to withstand:
●
●
●
high side 16.0 MPa (2320 PSI);
low side 12.0 MPa (1740 PSI);
high side 180ºC (356ºF) (discharge temperature).
Possible lubricants for the system
POE or PAG are the most likely options. Surplus oil will be stored in the accumulator and
sucked back to the compressor via an oil bleed hole in the accumulator through the suction
line. Refrigerant filters will most likely be applied in an R744 system, one location could be
within the accumulator, another one in front of the expansion device.
Accumulator/Internal heat exchanger
The accumulator is required to store lubricant for the compressor operation. The internal heat
exchanger acts as a heat exchanger to increase cooling capacity which is required mainly at
high ambient air temperatures. The accumulator also ensures that no liquid refrigerant enters
the compressor during the system operation.
Daly-01.qxd
3/18/06
48
10:43 PM
Page 48
Automotive Air-conditioning and Climate Control Systems
Figure 1.57 Mutli zone HVAC unit for CO2 application
(© 2005 Visteon All Rights Reserved)
Gas cooler
The efficiency of the system’s operation is highly dependent on the air flow through the gas
cooler ensuring enough heat is removed or absorbed.
Expansion valve
A solenoid operated valve which can be operated by a high frequency pulse width modulated
signal or an analogue DC voltage.
Visteon multi zone modular HVAC system
The modular multi zone HVAC system offers manufacturers the flexibility to personalise the
HVAC system depending on model specification and provides up to four temperate controlled
zones from one unit (this number of temperature controlled zones is usually provided by two
or more units). Modular units allow for mass production of common components gaining efficiencies of scale while still providing flexibility to the customer.
System benefits:
●
●
●
●
●
●
●
●
All metal sealing promotes no leak concept.
Cross platform usage – modular multi zone design.
Heat pump facility.
Full electronic control.
Low Global Warming Potential (GWP) (1) and zero Ozone Depleting Potential (ODP).
Improvement in fuel economy.
No additional fuel or electric heater required.
Reduction in emissions.
Negative aspects:
●
●
A higher level of technology is required and additional components are necessary.
Additional cost to suppliers exists due to large investment in research and development
(whether this cost will be passed on to the consumer or absorbed by the OEM is speculation).
Daly-01.qxd
3/18/06
10:43 PM
Page 49
Air-conditioning fundamentals
●
49
There will be an impact on the service and training industry requiring new knowledge, skills,
equipment and possibly a certificate of competence to service and repair such systems.
Absorption refrigeration
The absorption refrigeration cycle is attractive when there is a source of inexpensive or waste
heat readily available.This cycle uses a refrigerant that is readily soluble in a transport medium.
In brief, the condensation, expansion and evaporation processes are identical to those of the
vapour/compression cycle. But instead of the latter’s compression process, the absorption cycle’s
liquid transport medium absorbs the refrigerant vapour upon leaving the evaporator, creating
a liquid solution. This solution is then pumped to a higher pressure, and then heat is used to separate the refrigerant from the solution, whereupon the high pressure refrigerant flows to the
condenser to continue the familiar cycle. The equipment used to accomplish the solution/
dissolution processes is complex and heavy, but the advantage lies in the low work input requirement to raise the pressure of a liquid solution as compared to that required for compressing a
gas. If the heat utilised is otherwise wasted heat, the low operating costs of absorption systems can
be quite attractive.The two most common refrigerants used in absorption systems are ammonia,
with water as the transport medium, and lithium bromide in water. However, toxicity issues with
ammonia require safeguards, adding to system cost and complexity. Lithium bromide can be corrosive to most common materials, again adding to cost and complexity.Absorption systems are
used mostly in large non-vehicular building applications, though occasionally there has been
advocacy of their use as mobile air-conditioning systems.
Secondary loop system HFC152a
HFC152, which is flammable, must be used in a ‘secondary loop’ A/C system that uses a chiller
to transfer cooling from the refrigerant in the engine compartment to coolant that is circulating into the passenger compartment. The secondary loop is required as opposed to a primary
loop using a hydrocarbon-based refrigerant because if a leak occurs in the evaporator and
hydrocarbons are released then an explosion could occur.
The primary loop (refrigerant circuit) operates in the same manner as the vapour compression cycle using a hydrocarbon-based refrigerant instead of R134a. It is positioned under the
bonnet. The secondary system (coolant system) positioned inside the vehicle uses brine as a
cooling medium which is under pressure by the circulating front and rear pump to transfer the
heat from the front and rear coolers to the cooling medium. The reservoir is required to allow
for the expansion of the coolant. The system is a dual A/C system with front and rear coolers.
This system allows the easy addition of multiple cooling points with no additional expansion
device. The refrigerant charge in the primary system is about half when compared to an R134a
system with the same HVAC specification.
System benefits:
●
●
●
●
●
●
●
Wide choice of refrigerants can be used.
Enhances city traffic and idle cooling performance.
Potential for targeted cooling (e.g. seats).
Reduces refrigerant charge and leakage.
Potential for elimination of heater core resulting in smaller HVAC case and cost savings
(single heat exchanger for heating and cooling).
Refrigerant NVH reduction – front and rear.
Eliminates refrigerant maldistribution (coolant exhibits more uniform temperature
distribution than refrigerant).
Daly-01.qxd
3/18/06
50
10:43 PM
Page 50
Automotive Air-conditioning and Climate Control Systems
Refrigerant System
Variable
Displacement
Compressor
Accumulator
Coolant System
Reservoir
Ex. Device
Condenser
Rear
Pump
Chiller
Rear
Cooler
Front Pump
Front Cooler
Figure 1.58 HFC152a secondary loop system with front and rear cooler
(courtesy of Volvo)
System drawbacks:
●
●
●
●
The use of flammable refrigerant.
Cost of the system.
Energy penalty.
Weight of the system.
Gas refrigeration
The gas refrigeration cycle
In the past the gas refrigeration cycle, which is common on aircraft, has been considered for
the automobile industry. Research on the use of an air refrigeration system exists and cannot
be excluded as a future option due to this fact.
The gas refrigeration cycle is appropriately named due to the refrigerant remaining in a
gaseous state throughout the entire cycle. R729, otherwise known as air, is used as the refrigerant medium.
The pressure enthalpy diagram for a closed gas refrigeration system operates outside of the
phase transition of the refrigerant so that the saturation curves do not appear on the diagram
unlike the vapour compression cycle.
To aid the explanation of the system operation a Jaguar aircraft has been used for illustration (see Figure 1.59).
Daly-01.qxd
3/18/06
10:43 PM
Page 51
Air-conditioning fundamentals
51
Figure 1.59 Jaguar aircraft
(courtesy of RAF Coltishall)
Figure 1.60 Gas turbine engine showing the outlet feed to the A/C system
(courtesy of RAF Coltishall)
The refrigerant (air) enters a rotary compressor to raise its pressure and enthalpy. Air
charge is taken from both the compressors via an outlet as shown circled in Figure 1.60.
Only compressed gas at a temperature of about 190°C enters the A/C system because the
feed is situated on the outlet of the compression stage and not ignition.
The air then travels to a primary heat exchanger where it gives up heat at constant pressure.
The exchanger has ram air flowing through it to reduce the temperature of the air feed from
the gas turbine engines.
In Figure 1.61 the primary heat exchanger is fitted along the spine of the aircraft (large
circle) and is fed from the gas turbine engine (small circle).
The gas then flows to a turbine unit (cold air unit) where it expands reducing in enthalpy,
temperature and pressure. Some units connect the turbine to a compressor to recover some of
the energy given up during isenthalpic expansion. The air is at a temperature of about 100°C
upon leaving the turbine (cool air unit).
Upon leaving the turbine the gas flows to a secondary heat exchanger, situated behind the
cockpit. Cabin air circulates through the fins of the heat exchanger releasing heat and then a
water extractor removes moisture in the air.
Daly-01.qxd
3/18/06
52
10:43 PM
Page 52
Automotive Air-conditioning and Climate Control Systems
Figure 1.61 Exposed airframe
(courtesy of RAF Coltishall)
Figure 1.62 Secondary heat exchanger inlet
(courtesy of RAF Coltishall)
The air is then distributed around the cabin and auxiliary equipment situated on the aircraft. Because the cabin is pressurised, air is bled through two discharge valves in the aircraft
fuselage. This means that the system is an open A/C system because the air does not flow back
to the compressor to flow through the process again unlike a closed system.
The system can be operated manually or automatically to control the internal temperature of
the cockpit. Heating is achieved by bypassing the heat exchangers thus transferring heat laden
gas to the air distribution system. This is electronically sensed using thermistors and directed
using control flaps.
Evaporative cooling
The latent heat of vaporisation of water can provide cooling to vehicle occupants. A crude
approach is to spray one’s face with a water mist, then place the head outside the window of a
moving vehicle into the free air stream – the evaporating water carries away heat from the skin.
There have been devices, mostly in the 1950s – ‘The Weather Eye’ – that worked on evaporative
Daly-01.qxd
3/18/06
10:43 PM
Page 53
Air-conditioning fundamentals
53
Figure 1.63 Secondary heat exchanger
(courtesy of RAF Coltishall)
cooling.They consisted of a box or cylinder fitted to the window of the vehicle.The intake of the
unit would allow air to enter from outside and travel through a water soaked wire mesh grille
and excelsior cone inside the unit. The water would evaporate due to absorbing the heat in the
air and travel through the outlet of the unit which acted as a feed to the inside of the vehicle. At
one time, these devices had a certain attractiveness, particularly in hot, low humidity regions like
the US southwest. However, their performance compared to modern vehicular air-conditioning
systems is generally inadequate and they currently do not have significant popularity.
Thermo electric cooling (Peltier effect)
The basic concept behind thermoelectric (TE) technology is the Peltier effect – a phenomenon
first discovered in the early 19th century. The Peltier effect occurs whenever electrical current
flows through two dissimilar conductors; depending on the direction of current flow, the junction of the two conductors will either absorb or release heat.
Semiconductor material, usually bismuth and telluride, are generally used within the thermoelectric industry. This is due to the type of charge carrier employed within the conductor (see
Chapter 3). Using this type of material, a Peltier device can be constructed. In its simplest form
it consists of a single semiconductor ‘pellet’ which is soldered to electrically conductive material on each end (usually plated copper). In this ‘stripped-down’ configuration (Figure 1.65), the
second dissimilar material required for the Peltier effect is the copper connection which also acts
as a conductor for the power supply.
N type
Once impurities are added to a base material their conductive properties are radically affected.
For example, if we have a crystal formed primarily of silicon (which has four valence electrons),
but with arsenic impurities (having five valence electrons) added, we end up with ‘free electrons’
which do not fit into the crystalline structure. These electrons are loosely bound. When a voltage is applied, they can be easily set in motion to allow electrical current to pass. The loosely
bound electrons are considered the charge carriers in this ‘negatively doped’ material, which
is referred to as ‘N’ type material. The electron flow in an N type material is from negative to
positive. This is due to the electrons being repelled by the negative pole and attracted by the
positive pole of the power supply.
Daly-01.qxd
3/18/06
54
10:43 PM
Page 54
Automotive Air-conditioning and Climate Control Systems
P type
It is also possible to form a more conductive crystal by adding impurities which have one less
valence electron. For example, if indium impurities (which have three valence electrons) are
used in combination with silicon, this creates a crystalline structure which has ‘holes’ in it, that is,
places within the crystal where an electron would normally be found if the material was pure.
These so called ‘holes’ make it easier to allow electrons to flow through the material with the
application of a voltage. In this case,‘holes’ are considered to be the charge carriers in this ‘positively doped’ conductor, which is referred to as ‘P’ material. Positive charge carriers are repelled
by the positive pole of the DC supply and attracted to the negative pole; thus ‘hole’ current flows
in a direction opposite to that of electron flow.
Figure 1.64 shows two dissimilar materials, one is N type and the other P type. It is important to note that the heat will be moved (or ‘pumped’) in the direction of charge carrier movement throughout the circuit (it is the charge carriers that transfer the heat). Thus the electrons
flow continuously from the negative pole of the voltage supply, through the N pellet, through
the copper tab junction, through the P pellet, and back to the positive terminal of the supply.
The positive charge carriers (i.e. ‘holes’) in the P material are repelled by the positive voltage
potential and attracted by the negative pole and flow through the positive pole through the
P pellet and copper tab to the N pellet and the negative terminal. When a DC current is applied
heat is moved from one side of the device to the other – where it must be removed with a heat
Released
heat
Released
heat
N
P
Electron
flow
‘Hole’
flow
ⴚ
ⴙ
Absorbed
heat
Absorbed
heat
Figure 1.64 One simple semiconductor pellet
(courtesy of Tulleride)
Fan circulating air inside box
Heatsink inside box
TE device
Heatsink outside box
Fan circulating ambient air
LOW RES.
Conceptual Drawing of Air-to-Air
Thermoelectric Cooling Systerm
Figure 1.65 Peltier module
(courtesy of Tulleride)
Daly-01.qxd
3/18/06
10:43 PM
Page 55
Air-conditioning fundamentals
55
sink. The ‘cold’ side is commonly used to cool. If the current is reversed the device makes an
excellent heater.
Arranging N and P type pellets in a ‘couple’ and forming a junction between them with a
plated copper tab, it is possible to configure a series circuit which can keep all of the heat moving in the same direction. Using these special properties of the TE ‘couple’, it is possible to team
many pellets together in rectangular arrays to create practical thermoelectric modules. These
devices can pump appreciable amounts of heat, and with their series electrical connection, are
suitable for commonly available DC power supplies.The most common TE modules in use connect 254 alternating P and N type pellets and use 12 to 16 VDC supply and draw 4 to 5 amps.
Thermoelectric modules look like small solid-state devices that can function as heat pumps.
A ‘typical’ unit is a few millimetres square to a few centimetres square. It is a sandwich formed
by two ceramic plates with an array of small bismuth telluride cubes (‘couples’) in between.
Heatsinks are used to either collect heat (in heating mode) or dissipate collected heat into
another medium (air, water). Heat must be transferred from the object being cooled (or heated)
to the Peltier module and heat must be transferred from the Peltier module to a heatsink. Systems
are often designed for pumping heat from both liquids and solids. In the case of solids, they are
usually mounted right on the TE device; liquids typically circulate through a heat exchanger
(usually fabricated from an aluminium or copper block) which is attached to the Peltier unit.
Occasionally, circulating liquids are also used on the hot side of TE cooling systems to effectively dissipate all of the heat.
Peltier device cooling and heating speeds can change temperatures extremely quickly, but to
avoid damage from thermal expansion the rate of change is controlled to about 1°C per second.
It is theoretically possible to get a temperature difference across a Peltier module of 75°C
although it has been stated that in practice this is not achieved. In practice the results are about
half this figure.
If a TE module is to be used to cool anywhere near freezing then water condensation must
be considered. Ever-present water vapour begins to drop out of the air at the ‘dew point’.This will
result in the TE module, and what it is being used to cool, to get wet. Moisture inside of the TE
module will cause corrosion and can result in a short-circuit. A solution to this problem is to
operate the TE module in a vacuum (best) or a dry nitrogen atmosphere.
System control
Varying the power supply is often used. Pulse width modulation can be used, but a frequency
above 2 kHz is recommended and the voltage applied must not exceed the recommended
maximum voltage (Tmax).
Peltier devices are best suited to smaller cooling applications. They can be stacked to achieve
lower temperatures, but are not very ‘efficient’ as coolers due to the heating effect of the current flowing as well as drawing a great deal of current but act as very good heaters. This disadvantage can be offset by the advantages of no moving parts, no Freon refrigerant, no noise,
no vibration, very small size, long life, and the capability of precision temperature control.
Automotive application – Amerigon’s proprietary CCS system
Vehicle cabin air is drawn into the cushion and back TE modules and, based on inputs from
individual seat controls and from temperature sensors, the unit will either add heat or remove
heat to the air flow.
The basis of the system is the Peltier circuit. The Peltier circuit, heatsink (heat exchanger)
and fan assembly are mounted as one module.Air is used as a medium to move heat around the
seat through the perforated seat layers. Conditioned air is ported to the top surface of the foam
Daly-01.qxd
3/18/06
56
10:43 PM
Page 56
Automotive Air-conditioning and Climate Control Systems
through channels which evenly distribute the conditioned air over the surface. Breathable trim
covers allow the conditioned air to pass through to the occupant.When the Peltier device is cooling, heat is generated on the opposite side of the device which must be removed to allow the
temperature differential to exist.
This heat is pumped into the cabin space and is labelled on Figure 1.66 as ‘waste heat’. This
will create an extra load on the A/C system if being operated to cool the interior space.
When voltage is applied to the Peltier module in one direction one side of the Peltier device
will be hot and the other cool due to the direction of the charge carriers creating a T across
the Peltier device. Switching polarity of the circuit creates the opposite effect.
Figure 1.66 Amerigon’s Peltier module CCS system
(courtesy of Amerigon Corporation)
Figure 1.67 TE module showing Peltier couples and copper tabs stacked in series
(courtesy of Amerigon Corporation)
Daly-01.qxd
3/18/06
10:43 PM
Page 57
Air-conditioning fundamentals
57
Amerigon’s Peltier module proprietary CCS system allows occupants to select seat temperatures to promote comfort and reduce driver fatigue through the use of a solid-state heat pump
combined with an active, microprocessor controlled temperature management system to vary
heating and cooling capacity.
Amerigon states that it is the first to have successfully packaged this technology for use in
automotive seating applications.
1.6 The air-conditioning system
System activation
The signal to activate the air-conditioning system comes from the occupant(s). Activation is
completed by the onboard Electronic Control Unit (ECU). The ECU has a number of inputs
which send electronic signals based on sensed conditions, e.g. temperatures, pressures, speeds,
positions. Based on this information the ECU will either activate or deactivate (if already
operating) the system. If the system does not activate then a fault in the form of a code will be
stored in the computer and on some systems a light will be activated to tell the driver a fault
exists with the system. Advanced systems may use telematics to send a signal to a call centre
who will advise the customer of the required action, i.e. urgency on visiting a dealership.
Activation of the air-conditioning system is achieved under some of the following conditions:
●
●
●
●
●
●
The outside air temperature is above 9°C.
The engine has been running for more than 5 seconds.
The temperature of the evaporator is above 4°C (no ice forming over the surface).
The engine coolant temperature is approximately between 40°C and 105°C.
The vehicle is not rapidly accelerating or the engine is under high load (overtaking etc.).
The air-conditioning activation button has been selected and the interior fan is on.
Figure 1.68 Compressor drive system
(courtesy of Rover Group PLC)
Daly-01.qxd
3/18/06
58
●
●
10:43 PM
Page 58
Automotive Air-conditioning and Climate Control Systems
The sensors in the air-conditioning system have acknowledged that the system is under
pressure assuming that a quantity of refrigerant exists inside the system and that it has not
leaked out to the atmosphere (sensed by either pressure switches or sensors).
No fault codes exist in the ECU.
Simple electronic circuit
Upon activation current flows from the vehicle battery through fuses, switches fitted to the
A/C system and often an A/C relay to a magnetic clutch. The A/C relay is generally controlled
by an onboard computer, which makes the ultimate decision to allow the A/C system to be
activated based on system integrity, that is, the system has no faults and conditions are right for
system activation. The clutch is positioned behind the compressor pulley and once activated
will make a physical connection between the compressor pulley, which is driven by the engine,
and the internal pumping elements.
1.7 The expansion valve system
The air-conditioning system works on a continuous cycle. A compressor receives low pressure
heat laden refrigerant vapour from the evaporator. The compressor pressurises the refrigerant
Figure 1.69 Expansion valve system – also reproduced in colour in the plate section
(with the agreement of Toyota (GB) PLC)
Daly-01.qxd
3/18/06
10:43 PM
Page 59
Air-conditioning fundamentals
59
from 30 psi to approximately 213 psi depending on system demand. This increases the temperature from approximately 0 to 80°C. At this temperature and pressure the refrigerant is above
its boiling point of approximately 57°C. The compressor discharges superheated refrigerant
vapour to the condenser.
The refrigerant flows into the condenser. The condenser has numerous cooling fins in which
the vapour is pumped. In the condenser the high pressure vapour condenses into a high pressure liquid. This is achieved by reducing the temperature from, for example, 80°C to below
57°C which is the refrigerant’s boiling point. This is achieved by forcing air over the surface of
the condenser enabling heat to transfer from the refrigerant to the outside air thus reducing its
temperature (subcooled). Only refrigerant in the form of a high pressure subcooled liquid
leaves the bottom of the condenser outlet.
The subcooled liquid refrigerant flows into the receiver drier which stores, dries and filters
the liquid refrigerant.
The subcooled liquid refrigerant then flows from the receiver drier to the expansion valve
which then changes the refrigerant into low pressure low temperature liquid/vapour. This is
achieved by lowering the pressure using a variable orifice. The orifice has high pressure one
side (from the receiver drier) and low pressure the other (evaporator and compressor) and
allows a small quantity of refrigerant to flow through it. The sudden drop in pressure and temperature causes some of the refrigerant to vaporise which is called a flash gas.The low pressure
low temperature liquid/vapour then flows to the evaporator where the heat is transferred from
its surface to the refrigerant through vaporisation. The heat comes from either inside (recycled
air) or outside (fresh intake of air) the vehicle and is blown over the evaporator’s surface.
Once the refrigerant has completely vaporised and reached its saturation point it should still
be able to carry more heat. The refrigerant continues to flow through the remainder of the
evaporator coils absorbing more heat and becoming slightly superheated.
The low pressure low temperature slightly superheated vapour refrigerant flows to the compressor and the cycle repeats it self.
Note – temperatures are approximate and are dependent on refrigerant type, system load,
pressure and temperature relationship.
1.8 The fixed orifice valve system (cycling clutch orifice tube)
The air-conditioning system works on a continuous cycle. A compressor receives low pressure
heat laden refrigerant vapour from the evaporator. The compressor pressurises the refrigerant
from 30 psi to approximately 213 psi depending on system demand. This increases the temperature from approximately 0 to 80°C. At this temperature and pressure the refrigerant is above
its boiling point of approximately 57°C. The compressor discharges superheated refrigerant
vapour to the condenser.
The refrigerant flows into the condenser. The condenser has numerous cooling fins in which
the vapour is pumped. In the condenser the high pressure vapour condenses into a high pressure liquid. This is achieved by reducing the temperature from, for example, 80°C to below
57°C which is the refrigerant’s boiling point. This is achieved by forcing air over the surface of
the condenser enabling heat to transfer from the refrigerant to the outside air thus reducing its
temperature (subcooled). Only refrigerant in the form of a high pressure subcooled liquid
leaves the bottom of the condenser outlet.
The liquid refrigerant then passes through a fixed orifice tube – a tube with a fixed crosssectional area allowing only a metered quantity of liquid refrigerant to pass through it.
Daly-01.qxd
3/18/06
60
10:43 PM
Page 60
Automotive Air-conditioning and Climate Control Systems
1
2
1. Condenser
2. Compressor
3. Accumulator
4. Interior fan for
evaporator
5. Evaporator
6. Fixed orifice valve
7. Condensor fan
A High pressure liquid
B Low pressure liquid
C Low pressure gas
A D High pressure gas
B
C
D
6
7
4
5
3
7694/54/VF
Figure 1.70 Fixed orifice valve system
(reproduced with the kind permission of Ford Motor Company Limited)
7
A
B
C
D
E
F
6
5
1
4
3
2
8
10
7848/09/ESG
12
11
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
A
B
C
D
E
F
Condenser
Compressor
Accumulator/drier
Front heater, A/C blower
Expansion valve
Rear evaporator
Rear heater, A/C blower
Front evaporator
Fixed orifice tube
Low pressure switch
High pressure switch
Auxiliary fan
High pressure, liquid and warm
Low pressure, liquid and cool
Low pressure, gaseous and cold
High pressure, gaseous and hot
Cold air
Warm air
9
Figure 1.71 Dual air-conditioning system
(reproduced with the kind permission of Ford Motor Company Limited)
The low pressure low temperature liquid is then forced to expand rapidly due to the increase
in volume of the evaporator. This drop in pressure causes the refrigerant to boil (vaporise) and
absorb large amounts of heat energy which is transferred from the air flowing over the evaporator’s fins to the evaporator’s surface and thus to the refrigerant through vaporisation. After
the heat is removed from the air it is directed to the vehicle’s interior.
The low pressure low temperature liquid/vapour refrigerant flows from the evaporator to
the top of the accumulator which acts as a drier and storage device and separates any liquid
from vapour to protect the compressor (compressor can only pressurise vapour). The large
surface area of the accumulator also assists in any final evaporation of liquid refrigerant. The
saturated vapour and a tiny percentage of liquid refrigerant to carry oil from the oil bleed leave
the accumulator from the top and flow under low pressure to the compressor (suction side) and
the cycle repeats itself.
Daly-01.qxd
3/18/06
10:43 PM
Page 61
Air-conditioning fundamentals
61
4
1
2
3
5
6
7
9
8
ST/254/26
1. Condenser
2. High-pressure switch
3. Fixed orifice tulxe
4. Front evaporator
5. Suction accumulator/drier
6. Low-pressure switch
7. Rear evaporator
8. Expansion valve
9. Compressor
Figure 1.72 The components of a dual A/C system
Note – temperatures are approximate and are dependent on refrigerant type, system load,
pressure and temperature relationship.
1.9 Dual air-conditioning
This system often combines the use of a fixed orifice valve and an expansion valve. The primary operation of the system shown in Figure 1.71 is a fixed orifice system with additional outlets and inlets which feed an additional expansion valve and evaporator to aid additional
cooling. The additional cooling is often fitted in the back of a large multi-passenger vehicle
given two temperature control zones – front zone and rear zone. The outlet (8) from the primary system to the additional expansion valve/evaporator is feed from the inlet (7) to the fixed
orifice valve. Refrigerant splits and flows from the outlet of the primary system to an expansion valve and evaporator. Their function is identical to the normal expansion valve system.
Refrigerant vapour flows from the evaporator in the secondary system to the accumulator (4)
then (3) in the primary system. The accumulator ensures that hot liquid enters the compressor
from both systems.